"modern" API, take cad.3

[CAD97]

Closes #13. See #13 for more details.

[CAD97]

For clarity, since most of the code moved around and was adjusted: API design is by me, implementation code is directly adapted from the cleanup branch. I'd still suggest expanding package.authors to the authors credited in annotate-snippets, codespan, language-reporting, and retort:

authors = [
    "Brendan Zabarauskas <bjzaba@yahoo.com.au>",
    "Christopher Durham (CAD97) <cad97@cad97.com>",
    "Yehuda Katz <wycats@gmail.com>",
    "Zibi Braniecki <gandalf@mozilla.com>",
] # alphabetical order

but that's directly up to the package maintainer. All code in this PR is either original or adapted from this package, though API design is inspired by all of the above.

[epage]

We did a major re-work of the API in #90 and #91.

While there are ideas that could be broken out into their own Issue here, I'm going to close this as this form of the API isn't likely to move forward.