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.
Merge branch 'force-ext' of github.com:MaxPower15/docco into ext-cleanup
Cleanup submission from @MaxPower15, that adds support for specifying the file type of the inputs based on a given extension other than the one the file has on disk.
Missing README update for -e flag.
Merge branch 'ext-cleanup'