Refactor generation

* Split generate.py to generate.py and interactive.py and refactor code

The main motivation behind these changes is to try to decorrelate use
cases in order to implement future improvements such as unk replacement
with original string during evaluation on test and writing predictions
to output file.
The previous implementation worked well but I found it difficult to
integrate these future improvements.

* Add --replace-unk arg to be used without align dict

Replacing <unk> tokens can be beneficial even without an alignment
dictionary.