Add support for more file extensions.

"gyp", "gypi", "gn" are for build files in Chrome.
"mk" is for makefiles.
"mm" is for objective-C++ files.
"pch" is for pre-compiled header files.
"plist" is for xml-like files on OS X.
Merge pull request #216 from Jyaif/patch-1

Add support for more file extensions.