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).
Should not be tied to Haxe but defined as a flag in the context features.