Add TestNearest support for Lua's Busted test runner

[mblayman]

This PR updates the busted plugin to add support for TestNearest so that it will execute the nearest test instead of the entire file (which is the current implementation). The modifications handle both it and describe functions. I also wrote some tests for Moonscript to make sure that it works for that style of Lua too.

I made this work by taking advantage of the --filter flag with busted. The --filter flag expects the value to be a proper Lua pattern. Because of that constraint, the plugin does the proper escaping of the Lua pattern "magic characters" (defined in the Lua docs).

There is a small limitation to this implementation. If multiple tests in the same file have the same string description, then TestNearest will run all identically labeled tests. This seemed like an acceptable tradeoff because it's still more accurate than running all the tests in a file.

PR Checklist:

• Add fixtures and spec when implementing or updating a test runner
• Update the README accordingly
• Update the Vim documentation in doc/test.txt

[codeinabox]

Thank you for fixing this typo!

[codeinabox]

Thank you for taking the time for this great addition, it's super appreciated!