Add install-bash-completion and install-shell-completion targets

[strk]

I've only done bash as I use bash and have no idea how zsh and fish deal with custom completion files

[strk]

The completion seems to be not working with the click branch, will check if it works with the master branch

[ihabunek]

It's only implemented on the click branch. Not yet merged to master. It works for me on zsh.

[strk]

For bash it fails:

eval "$(_TOOT_COMPLETE=bash_source toot)"
Unknown command '-'
Run toot --help to show a list of available commands.
v0.39.0: command not found
Authentication:: command not found
bash: syntax error near unexpected token `('

Is this something supposed to be implemented by click itself ?

[strk]

It looks like _TOOT_COMPLETE env variable has no effects at all on toot:

$ _TOOT_COMPLETE=bash_source toot
toot - a Mastodon CLI client
v0.39.0

Authentication:
  toot login            Log into a mastodon instance using your browser (recommended)
...

I guess it would be a lot safer to just build the completion scriptsi on make and install the generated output, rather than calling the dangerous eval on every new shell

[strk]

this PR continues in #446