Add convert rule for message context.

[Javascript Gettext](http://jsgettext.berlios.de/) has [context feature](http://jsgettext.berlios.de/doc/html/Gettext.html#pgettext__msgctxt__msgid__) like [GNU gettext](http://www.gnu.org/software/gettext/). of course, [`po2json`](http://jsgettext.berlios.de/doc/html/po2json.html) can parse `msgctxt` properly.[check [this demo page](http://jsgettext.berlios.de/demo/index.html).]

This PR implements its feature.(it just put separator(`\u0004`) between `msgctxt` and `msgid` and using it for key.)