-s/--setting x y gets merged into datasette.yml, refs #2143, #2156

This change updates the `-s/--setting` option to `datasette serve` to allow it to be used to set arbitrarily complex nested settings in a way that is compatible with the new `-c datasette.yml` work happening in:
- #2143

It will enable things like this:
```
datasette data.db --setting plugins.datasette-ripgrep.path "/home/simon/code"
```
For the moment though it just affects [settings](https://docs.datasette.io/en/1.0a4/settings.html) - so you can do this:
```
datasette data.db --setting settings.sql_time_limit_ms 3500
```
I've also implemented a backwards compatibility mechanism, so if you use it this way (the old way):
```
datasette data.db --setting sql_time_limit_ms 3500
```
It will notice that the setting you passed is one of Datasette's core settings, and will treat that as if you said `settings.sql_time_limit_ms` instead.

docs/cli-reference.rst

@@ -113,8 +113,8 @@ Once started you can access it at ``http://localhost:8001``
                                       /MOUNT/...
       --memory                        Make /_memory database available
       -c, --config FILENAME           Path to JSON/YAML Datasette configuration file
-      --setting SETTING...            Setting, see
-                                      docs.datasette.io/en/stable/settings.html
+      -s, --setting SETTING...        nested.key, value setting to use in Datasette
+                                      configuration
       --secret TEXT                   Secret used for signing secure values, such as
                                       signed cookies
       --root                          Output URL that sets a cookie authenticating