Allow custom language definitions

[AndyMcAliley]

Fixes #789

This PR adds a second duplicate --option directive to the ctags command, using deffile twice. The first --option comes prior to --language-force and lets ctags know the custom language. The second --option comes last to override the default --kinds-<lang> definition.

This PR addresses the same problem that #792 addresses, but in a different way that doesn't require additional user options.

This fix needs testing, especially on the use case described in #776.

[AndyMcAliley]

Actually, maybe it would be even better to check if the language is recognized by ctags, and only include the first --option directive for custom languages. I added a commit to do that. I added another commit to only call ctags --list-languages once.