Issue 2052 escape double and triple braces

This implements the feature request in #2052. It adds two new ```TemplateTags```, ```ESCAPEDDOUBLE``` and ```ESCAPEDTRIPLE```. The tags get parsed and store the text between the braces in ```tag.value```. The compiler then outputs the tags using ```HTML.Raw```.

Since this is my first time diving into Meteor core, I wanted to take it a little slow and get some feedback on my decisions. 

* Is adding new tags the best way of approaching this? It seems the simplest and cleanest way to me.
* Is ```HTML.Raw``` the best way of outputting the strings?

I can also write up some more tests to try to cover more edge cases. The ones I did are very simple.