support for prepend and append

[minimit]

with

// @codekit-prepend path/to/file.js
// @codekit-append path/to/file.js

As I explained it's the syntax this 'precompiler' sofwares use. #30

The lines added are very simple, I just:

• added one regex for prepend and one regex for append (you should be able to add your own if you want)
• executed expand() with the new argument regex
• added directive checks
• with the functions prependString() and appendString() I cached the results, that I add to newText during buffer.

Also I added optional [] on multiple files for this syntax

// @codekit-prepend "path/to/file.js", "path/to/file.js"

[minimit]

I don't know how to run the tests. Will look into it if it's needed.

[pixelfuture]

This is brilliant. Can we get this committed? This is exactly what I need.

[KenEucker]

All that this pull request needs now is some tests created around the use cases for this feature. I will circle back to this item after we get through the more critical PRs and resolve the conflicts if I need to.