"datasette install" and "datasette uninstall" commands

[simonw]

When installing Datasette plugins it's crucial that they end up in the same virtual environment as Datasette itself.

It's not necessarily obvious how to do this, especially if you install Datasette via pipx or homebrew.

Solution: datasette install datasette-vega and datasette uninstall datasette-vega commands that know how to install to the correct place - a very thin wrapper around pip install.

[simonw]

Prototyped in this thread: #335 (comment)

[simonw]

$ datasette install --help
Usage: datasette install [OPTIONS] PACKAGES...

  Install Python packages - e.g. Datasette plugins - into the same
  environment as Datasette

Options:
  --help  Show this message and exit.
$ datasette uninstall --help
Usage: datasette uninstall [OPTIONS] PACKAGES...

  Uninstall Python packages (e.g. plugins) from the Datasette environment

Options:
  -y, --yes  Don't ask for confirmation
  --help     Show this message and exit.

[simonw]

This will simplify the instructions for installing plugins with Datasette install via homebrew, refs #923