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.

update-docs-help.py

@@ -0,0 +1,25 @@
+from click.testing import CliRunner
+from datasette.cli import cli
+from pathlib import Path
+
+docs_path = Path(__file__).parent / "docs"
+
+includes = (
+    ("serve", "datasette-serve-help.txt"),
+    ("package", "datasette-package-help.txt"),
+    ("publish", "datasette-publish-help.txt"),
+)
+
+
+def update_help_includes():
+    for name, filename in includes:
+        runner = CliRunner()
+        result = runner.invoke(cli, [name, "--help"], terminal_width=88)
+        actual = "$ datasette {} --help\n\n{}".format(
+            name, result.output
+        )
+        open(docs_path / filename, "w").write(actual)
+
+
+if __name__ == "__main__":
+    update_help_includes()