Add string interpolation

[DleanJeans]

For issue #61
This passes all the tests I wrote over DleanJeans/hscript-plus.
Let me know if there's something wrong!

[ncannasse]

Could you add some unit tests for it in Test.hx?

[DleanJeans]

I ran into a problem with string interpolation inside another ('${'string'}'). I'll need to modify Parser::readString() and it's gonna take a while. Would you mind helping me by explaining how string interpolation exactly works in Haxe itself?

[Simn]

Do we really want this in hscript? String interpolation is an order of magnitude more complex than everything else, especially when it starts being nested. We haven't even solved that in the compiler properly.

[DleanJeans]

I guess I'll just leave it unsolved.

[DleanJeans]

Will this be merged any time soon? If not, at least I can leave it as a feature in my hscript-plus.

[ncannasse]

Unless it's well tested and compliant with Haxe I think it shouldn't be merged.