Add Haxe string interpolation completion, closes #173

Works even with nested expression blocks like

```haxe
'${'${expr}'}'
``` 

although the reformater currently messes those up.

![](http://i.imgur.com/BsjvNUX.gif)

I also made some adjustments to the behaviour `Ctrl+Space`, it now tries to be "contextual" (which is much more intuitive in my opinion).
Oh the code was checking the string type in the "wrong" direction initially!
That's strange...
I see the function is only used in one context - when pressing Shift+Enter inside a string and so to split it. I'm trying to remember who wrote that.
My gut feeling is that the forward version is wrong and we should only check the string type in reverse.

However there is a bug in the code: when going backwards, it doesn't skip properly escaped characters (`\"`).