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).
That was the point of this. I figured It should work when a closing quote is missing, as well, as you typically type from left to right and the "Add Closing Braces" option is false by default (is there any chance that that could be changed, btw?).