`po_` function for interactive translation

[hadley]

Extract the interactive workflow out of translate_packge() so it can be called individually. Not sure what to call it; probably needs to include interactive or similar in the name.

[MichaelChirico]

My current thinking is it should just be called po_translate() and it should flexibly accept:

(1) A .po file (2) A .pot file (3) A po/ directory with a .po file or .pot template (4) A package directory

(2)-(4) run the steps necessary to create a .po file