file type detection should be configurable

[brybacki]

As a user I want to turn off some file extensions from being handled by this tool, because other plugins are already handling them.

Specific case:
The Syntaxtic is coloring the .diff and .patch files, but I already have an extension "Git Patch Viewer" that -at least for me - is much better at handling my .patch files. The problem is that the Syntaxtic goes first when it detects .patch file, and then the Git Patch Viewer does not work on this file.