Observe & update draw options

[Techn1x]

This PR branches off of the 'initial-features' PR I also made here #9

I needed a way to change the options for new shapes that are drawn (specifically, I needed to change the colour). This is a first attempt at getting the 'draw' arg to be observed so that new shapeOptions can be applied, using the leaflet-draw method setDrawingOptions. Seems to work, I have only tested changing the colour though.

I also included a commented out section in _drawChanged() which will also apply the new draw shapeOptions to existing objects that were previously drawn. There are some caveats to that code;

• Not sure how to tell what shape a previously drawn object is (they're all instances of L.Polygon..), so it just gets the draw.initial.shapeOptions applied to it, this is probably not desired
• Users may not want existing shapes to be updated with new draw options - perhaps existing shapes should be updated in some other way? Or put this functionality behind an arg to the component called something like updateExistingShapes=true ?

My leaflet skills aren't the best and I've only been using this addon for a day so please chime in if you know any improvements that can be made (or just go ahead and commit them!)

[Techn1x]

Might be a better way to do this using ember-diff-attrs and didUpdateAttrs(), rather than observers
https://github.com/workmanw/ember-diff-attrs