Added G-Code (ISO 6983) support
Merge branch 'gcode' of github.com:adamjcook/highlight.js into gcode

Conflicts:
	AUTHORS.en.txt
	CHANGES.md
Adding Adam Joseph Cook to AUTHORS.ru.txt
Removed leading line-break from G-Code snippet.
Added detection test for G-Code
Added "gcode" as a canonical class name for G-Code.