Patch SciLexer to support basic string interpolation highlighting, closes #367

As you can see here, it doesn't work correctly with nested expression blocks and interpolation blocks that don't have any chars in between, but it should be good enough for the vast majority of use cases anyway.

![](http://i.imgur.com/RblA2BB.png)

```haxe
// working

trace('${i} '); //

trace('${i}'); //

trace('$i '); //

trace('$i'); //

trace('$i$i'); //

trace('value ="$i"'); //

// not working

trace('${'${10} '} '); //

trace('${'${10} '}'); // 

trace('${'${10}'}'); // 

trace('$i${5}$i'); //

trace('${i}${i}'); //
```
Increases the chance to be accepted in scintilla. We shoukd ask if this kind of feature could possibly be accepted...