Refs #417
First proof-of-concept for Datasette Library. Run like this:
datasette -d ~/Library
Uses a new plugin hook - available_databases()
BUT... I don't think this is quite the way I want to go.
If you start Datasette with no files, it will connect to :memory: instead.
When starting it with files you can add --memory to also get a :memory: database.
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.
Also introduced a new mechanism for ensuring the --help examples in the
documentation reflect the current output of the --help commands, via a new
update-docs-help.sh script. Closes#336