Add an --extensions flag to change valid file extensions.

The valid cpplint extensions are .h, .cpp, .cc, .cuh and .cu. It is
common for .hpp to be a valid extension to check, the --extensions flag
allows the user to specify which extensions they think are valid.

Patch by Matt Clarkson <mattyclarkson@gmail.com>

git-svn-id: http://google-styleguide.googlecode.com/svn@119 955884f1-7149-0410-9467-59e7ac3f4d80