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).
May give weird result for the last character - also you usually want to loop with: `prev = c; c = next; next = ...;`.
The pattern in the code is to generally to get characters using Sci.GetCharAt() instead of getting the line text.