[Markdown] Add GFM checkbox notation

[michaelblyons]

• Sublime Version: 4062
• OS Version: MacOS Catalina

Immediately following a list item marker -/+/*, match [ ], [X], and [x] as some kind of keyword or other entity. This is particularly so they don't get confused as link text identifiers when a real link follows (see below).

Integers
========
- [ ] [PEP 0237][]: Essentially, `long` renamed to `int`. That is, there is only one...
- [x] [PEP 0238][]: An expression like `1/2` returns a `float`. Use `1//2` to get the...
- [x] Octal literals are no longer of the form `0720`; use `0o720` instead.

[PEP 0237]: https://python.org/dev/peps/pep-0237
[PEP 0238]: https://python.org/dev/peps/pep-0238