Dynamic property access with [] operator

[justinbmeyer]

From: canjs/canjs#1157

Support dynamic property access with a [] operator.

This can be done by making an expression Type similar to Call: https://github.com/canjs/can-stache/blob/master/src/expression.js#L117

And then making sure it's parsed and created correctly around here: https://github.com/canjs/can-stache/blob/master/src/expression.js#L625