Add --languages flag

This feature lets you specify which audio languages to copy
and the order in which to copy them.

e.g. to make Japanese the first audio Language, specify it
first.  --languages="Japanese,English"
- Modify ragel to add audio track index value
- add Languages and an Order map to GomkvConfig
  - I may change the language order map into a method later
    to make it easier to test
- Add interfaces to make AudioMeta sortable by golang
- Update unit tests
- Add tests for sorting