Added stringOnly and singleFile options

Hello. Nice grunt tool you have there. More complete than the one I wrote. Too bad I found yours a little late. 

However, for yours to replace mine in the project I need it for, I need two options: the option to write several .po files to a single file; the option to write just the translation instead of [null, "string"]. Seriously, what is up with the null arrays? 

Anyway, I've created the options. I did not create a unit test, but it works for me. I would like you to merge the branch so I can create a dependency on your repo instead of mine. Are you OK with these changes?