Allow `mark_unknown` to be actually used in `translate`

[ygorg]

Fixes #82 The option for mark_unknown translation was present but not used. This code fixes that. It also clarifies the role of $1 and $2 (variable names come from apertium CLI) in parse_mode_file.

This function was untested for the wrapper because I couldn't make it work. For information: in #82 dimitarsh1 dealt with that by prevented using the wrapper when mark_unknown was used.