Remove the default file extension -> .html clobber

This makes it impossible to use gulp-mustache with multiple file
extensions, because there is no way to retrieve the current file
extension. This task is better handled by plugins like
gulp-rename, and the default for gulp-mustache should allow general
use of the template engine.
Merge pull request #4 from oconnore/extension-fix

Remove the default file extension -> .html clobber