spellcheck: remove to separate package

[dgw]

Despite switching from enchant to aspell (#1164), the spellcheck module can still cause issues with installing Sopel on Windows (#1142). Even on *nix systems, there are system-level package requirements (libraries and dictionary files) that often trip up the installation. It's more than a little absurd for one optional plugin to potentially halt the entire install process for Sopel, when the user might not even want any of the core module set.

I still want to leave most "package-ification" for a later release (see #1291), but I've become convinced over the past few months that removing spellcheck from the base installation package sooner is warranted. I just see way too many complaints about installation problems related to it, and spellcheck is the only core plugin with dependencies that pip can't always install.

So, if anyone wants to be co-maintainer of the soon-to-be-external spellcheck plugin, comment here and/or send me a PM on freenode (I'm also dgw there). When I return from my overseas trip in a week or so, I'll make the repository, add collaborators (if any), and get started on this.

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Tasks:

• Remove plugin and dependencies from core (PR open: spellcheck: remove plugin (moving to separate package) #1675)
• Create new package repo (done: sopel-irc/sopel-spellcheck)
• Extract spellcheck.py into its own package and test it (done: see above repo)
• Publish new package (will probably wait until at least an alpha of Sopel 7 is published, so pip can actually find a valid requirement candidate)

[dgw]

The plugin is now published at https://pypi.org/project/sopel-spellcheck/ (or see the repo).

As PyPI does not have any release of Sopel 7.x available yet, this requires an existing "editable" install of Sopel @ 40162a4 or newer. pip will not be able to satisfy the sopel>=7.0 dependency on its own until we begin publishing alpha/beta/RC versions.