Property name interpolation

Support for "property name interpolation" feature (36, #698).

Just a few comments on implementation: Actually I'm not too happy by the fact that I did not manage to reuse existing parse functions ($() and variableCurly() in particular). In fact I started to write all this with those functions in mind but then realized that most likely it will end with quite cluttered and iffy code because of whitespace backtracking. So I simply switched to this "self-contained" parsing method.