Add CI configuration and tests, perform simple fixes

[honzajavorek]

Implements and closes https://github.com/sphinx-contrib/emojicodes/issues/13. Doesn't make the tests pass, because I was unable to debug the following problems:

/Users/honza/Projects/emojicodes/docs/source/index.rst:5: WARNING: Inline emphasis start-string without end-string.
/Users/honza/Projects/emojicodes/docs/source/index.rst:5: WARNING: Inline emphasis start-string without end-string.
/Users/honza/Projects/emojicodes/docs/source/index.rst:5: WARNING: Inline emphasis start-string without end-string.

If those are fixed, I believe the tests will pass. Under https://github.com/honzajavorek/emojicodes/pull/1 it should be visible how it works.

The Windows tests will probably fail as well even when the other will pass, because I'd say make won't be installed out of the box, but that can be later fixed simply by choco install make I think. Let's see. It'd focus on making it pass on Linux/macOS now. Also, perhaps it doesn't make sense to test the docs on multiple OS'es and it's okay to test it just on Linux. We'd test on multiple OS'es only if we have real tests for the code. But so far I'm keeping what I have as a demonstration of what's possible if we make it pass, and we can expand on that / restructure it later.

[honzajavorek]

Oh, tox supports running arbitrary commands? I thought it's just for tests of code (e.g. pytest), that's why I skipped it for this stage (also, the wording used in https://github.com/sphinx-contrib/emojicodes/issues/13 made me think introducing tox could be a next step).

[Peque]

@honzajavorek Yeah, you are right interpreting #13. However, since it seems you are on a streak I thought maybe we could take advantage and hit two targets with one shot! :stuck_out_tongue_winking_eye:

[honzajavorek]

I'll check out tox, but no promises about timing :)