Add support for gettext style formats

We're using Pootle as a translation server which uses the gettext .po format.  To use this, we're using a format similar to what android2po (https://github.com/miracle2k/android2po) uses where the key is used as the msgctxt, the original string is the msgid, and the translation is the msgstr.

Making twine support the gettext .po allows us to easily integrate the Pootle output with both the iOS and Android builds.