Adding support for --force-ext option.

Allows the user to specify an extension on the commandline. When present, all input files will be processed as if they have this extension.

This is useful if you have filenames with multiple sequential extensions, or just a non-standard extension that cannot be changed for one reason or another.

For example, I have quite a few files which have `.coffee.erb` extensions. These files are 99.9% Coffeescript and can be parsed as such, but having the `.erb` suffix is required for our rendering engine. With this change, I can process them like so:

    docco --force-ext .coffee path/*.coffee.erb