Switch from invoke to click for CLI

[toolness]

We're currently using invoke for our CLI needs, but it has a number of disadvantages:

• It's pre-1.0, with the following red warning on its website:

  While fully usable, Invoke is still pre-1.0 software and has no backwards compatibility guarantees until the 1.0 release occurs!

  Given the current allocation of human resources devoted to keeping autoapi up-to-date, I have concerns about this.

• It doesn't seem to offer any way to offer help to command-line users, e.g. invoke --help doesn't even show a list of tasks, and it doesn't appear to be possible to document the tasks themselves.

Jacob Kaplan-Moss strongly recommends using click, and it appears to be stable with an active community. It also supports a lot of options for offering help to command-line users.

[toolness]

That said, since we're using Flask, I guess flask-script is another option.

[jmcarp]

No opinion on invoke vs click in this case--I think either will work fine. FWIW, invoke is more mature than the version number suggests--it was spun off fabric years ago, so it's been around for a while. But I wouldn't use flask-script, since the latest flask now includes click. My guess is that flask-script is going to go away once enough people have upgraded flask.