Syntax Highlighting for .gitignore Files in Atom

Adds syntax highlighting to .gitignore files in Atom.

Header Blocks

A "Header Block" is started and ended by a line that begins with # followed by any character(s) (except a space), and ended by another #. (See the gitignore sample file for an example of a valid "Header Block").

Screenshot

(Theme used in screenshot: "Oscuro Syntax"...which I have not yet finished but will soon)