Basic support for I18n contexts as requested by #2063

I've tried creating the basics for this. It can only read the translations using contexts (not extract them yet) and it only has the ```__x``` function for now (as proposed by @garas in issue #2063).
At some point we might want to transfer the 2nd...7th arguments into a single 2nd $options array?
Really starts to look like a nightmare.
Maybe for 3.x then?