Ensure --help examples in docs are always up to date, closes #336

Unit tests now check that docs/*.txt help examples are all up-to-date.

I ran into a problem here in that the terminal_width needed to be more
accurately defined - so I replaced update-docs-help.sh with update-docs-
help.py which hard-codes the terminal width.

docs/datasette-package-help.txt

@@ -1,19 +1,18 @@
 $ datasette package --help
 
- Usage: datasette package [OPTIONS] FILES...
+Usage: cli package [OPTIONS] FILES...
 
   Package specified SQLite files into a new datasette Docker container
 
 Options:
-  -t, --tag TEXT            Name for the resulting Docker container, can
-                            optionally use name:tag format
+  -t, --tag TEXT            Name for the resulting Docker container, can optionally use
+                            name:tag format
   -m, --metadata FILENAME   Path to JSON file containing metadata to publish
   --extra-options TEXT      Extra options to pass to datasette serve
   --branch TEXT             Install datasette from a GitHub branch e.g. master
   --template-dir DIRECTORY  Path to directory containing custom templates
   --plugins-dir DIRECTORY   Path to directory containing custom plugins
-  --static STATIC MOUNT     mountpoint:path-to-directory for serving static
-                            files
+  --static STATIC MOUNT     mountpoint:path-to-directory for serving static files
   --install TEXT            Additional packages (e.g. plugins) to install
   --spatialite              Enable SpatialLite extension
   --version-note TEXT       Additional note to show on /-/versions

docs/datasette-publish-help.txt

@@ -1,9 +1,8 @@
 $ datasette publish --help
 
- Usage: datasette publish [OPTIONS] PUBLISHER [FILES]...
+Usage: cli publish [OPTIONS] PUBLISHER [FILES]...
 
-  Publish specified SQLite database files to the internet along with a
-  datasette API.
+  Publish specified SQLite database files to the internet along with a datasette API.
 
   Options for PUBLISHER:     * 'now' - You must have Zeit Now installed:
   https://zeit.co/now     * 'heroku' - You must have Heroku installed:
@@ -12,8 +11,7 @@ $ datasette publish --help
   Example usage: datasette publish now my-database.db
 
 Options:
-  -n, --name TEXT           Application name to use when deploying to Now
-                            (ignored for Heroku)
+  -n, --name TEXT           Application name to use when deploying
   -m, --metadata FILENAME   Path to JSON file containing metadata to publish
   --extra-options TEXT      Extra options to pass to datasette serve
   --force                   Pass --force option to now
@@ -21,8 +19,7 @@ Options:
   --token TEXT              Auth token to use for deploy (Now only)
   --template-dir DIRECTORY  Path to directory containing custom templates
   --plugins-dir DIRECTORY   Path to directory containing custom plugins
-  --static STATIC MOUNT     mountpoint:path-to-directory for serving static
-                            files
+  --static STATIC MOUNT     mountpoint:path-to-directory for serving static files
   --install TEXT            Additional packages (e.g. plugins) to install
   --spatialite              Enable SpatialLite extension
   --version-note TEXT       Additional note to show on /-/versions

docs/datasette-serve-help.txt

@@ -1,27 +1,22 @@
 $ datasette serve --help
 
- Usage: datasette serve [OPTIONS] [FILES]...
+Usage: cli serve [OPTIONS] [FILES]...
 
   Serve up specified SQLite database files with a web UI
 
 Options:
   -h, --host TEXT           host for server, defaults to 127.0.0.1
   -p, --port INTEGER        port for server, defaults to 8001
-  --asgi                    Run in ASGI mode
   --debug                   Enable debug mode - useful for development
-  --reload                  Automatically reload if code change detected -
-                            useful for development
-  --cors                    Enable CORS by serving Access-Control-Allow-
-                            Origin: *
+  --reload                  Automatically reload if code change detected - useful for
+                            development
+  --cors                    Enable CORS by serving Access-Control-Allow-Origin: *
   --load-extension PATH     Path to a SQLite extension to load
-  --inspect-file TEXT       Path to JSON file created using "datasette
-                            inspect"
-  -m, --metadata FILENAME   Path to JSON file containing license/source
-                            metadata
+  --inspect-file TEXT       Path to JSON file created using "datasette inspect"
+  -m, --metadata FILENAME   Path to JSON file containing license/source metadata
   --template-dir DIRECTORY  Path to directory containing custom templates
   --plugins-dir DIRECTORY   Path to directory containing custom plugins
-  --static STATIC MOUNT     mountpoint:path-to-directory for serving static
-                            files
+  --static STATIC MOUNT     mountpoint:path-to-directory for serving static files
   --config CONFIG           Set config option using configname:value
                             datasette.readthedocs.io/en/latest/config.html
   --version-note TEXT       Additional note to show on /-/versions