sopel.cli.utils

[Exirel]

Related to #1471:

• create a sopel.cli.utils modules,
• functions from sopel.cli.utils are available from sopel.cli as shortcuts,
• find, enumerate, and load configurations functions are now in sopel.cli.utils,
• sopel.run_script now uses sopel.cli.utils for these functions,
• existing tests are moved around into test/cli/test_cli_utils.py for consistency,

and there is no new feature, only internal changes: everything is backward compatible.

From this PR, it is possible to change how settings are loaded (which files, from where, etc.), and there is one single place to load a Sopel's configuration.

[Exirel]

Oh, so in Py2, the subparser action is mandatory... interesting!

[dgw]

Once again, the majority of line notes are related to docstrings.

There are relatively more larger questions in this review, though. That's why it's a "Comment" instead of "Request changes"—the changes I might request aren't defined yet.

[Exirel]

@dgw should be good for one last review.

[dgw]