Improve CLI

[cthoyt]

Reference Issues/PRs

Closes #152

What does this implement/fix? Explain your changes.

This PR does several things to improve the CLI:

1. Replaces docopt with click (it's much easier to work with)
2. puts the selene_cli.py inside the package as cli.py
3. exposes the CLI using __main__.py so it can be run with python -m selene_sdk
4. exposes the CLI using the entry_points kwarg in setup.py so it can be run in the shell as selene_sdk
5. Improve the documentation related to the CLI to make this clear to anyone reading it later

Even though it switched from docopt to click, and also the location moved, everything should continue working exactly the same way. Click auto-generates the documentation for the CLI and I posit is better than docopt, but if you don't agree, I could also change it back.

Happy hacktoberfest / greetings from Germany!

What testing did you do to verify the changes in this PR?

It still does the same as before, now with just a slicker interface!

[kathyxchen]

@cthoyt Thank you so much for submitting this PR! I completely agree with the comments you made in the corresponding issue - it's a change we hadn't prioritized but will make the user experience of the library so much better, this is really awesome. :) I'll be sure to review your PR sometime this week.

[kathyxchen]

Thank you again for this contribution! I have one minor ask - can you add click (with whatever version you are using) to the selene-gpu.yml and selene-cpu.yml conda environment files? Just because some people will only use Selene locally (i.e. without setup.py install or conda install). I think we can also remove selene-gpu-snapshot.yml as well since the other two files are more usable. LGTM otherwise. :) I'll approve and push after those changes are made!

[cthoyt]

Done in cthoyt-forks-and-packages@58cf55f, cthoyt-forks-and-packages@e732528, and cthoyt-forks-and-packages@1aff5e2. Let me know if that's sufficient

[kathyxchen]

LGTM! Thank you so much for your contribution!