Support for ES6 Templates

This pull request is meant to add support for [ECMAScript 6 templates](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/template_strings) and address suggestion #13.

**Right now this branch supports:**
* Both simple and substitution templates for all emit targets
* Tagged templates for solely ES6 (and above) targets
* Syntactic highlighting in the LS for template literals

**What's left to do:**
* Testing - can't get enough of it.
* Type checking for tagged template expressions by checking assignability of substitution expressions to respective tag parameters.
* Add typings for the "cooked strings" parameter of tags.
* Improve emit with respect to unnecessary parentheses.
* Improve emit by omitting template empty tails (and maybe heads?).
* Experiment with indentation in the language service.
* Experiment with brace matching on substitution templates.


*Note: This is an unfinished pull request. While you should feel free to comment on code quality, efficiency, and potential edge cases, keep in mind that certain changes will be coming in periodically.*
add assert that you're in ES5 mode or lower.