Color support

[ForNeVeR]

Changes extracted from #138 by @B3zaleel.

Closes #165.

TODO:

• leave only the original commands (\color and \colorbox)
• fix the failing unit tests
• think about the accepted color formats
  • check if any existing TeX distributions have something like that
• write the documentation for the new color input
• add tests for the new input
• fix the implementation according to the tests
• make sure it's robust enough so the new color parsers could be added easily
• review all the comments below
• add tests for double/integer parse errors in the color parsers (all should throw TexParseExceptions)
• add tests for negative / too big values for all the numbered models (should also throw TexParseExceptions)
• make sure the corresponding infrastructure and helper classes are public
• review all the code one final time

[ForNeVeR]

@B3zaleel if you're going to make any changes — please create a new branch based on feature/color-support in this repository and push your changes to that branch. I'll move your new commits (if any) to this pull request manually.

And feel free to ask me any questions if something isn't clear to you about my change requests. I'll be glad to explain everything, produce examples or fix some of the issues myself (or even all of them).

For now, I'm awaiting either questions or changes from you. And, again, thanks for your interest in our project, we appreciate it very much! 👍

[ForNeVeR]

Alright, I'm going to finalize this task based on @B3zaleel's contributions and my comments. Stay tuned.

[ForNeVeR]

Thanks, everyone! Now this is (finally!) ready for merge.