Continuation of: Add Template Instrumentation (#103)

[NullVoxPopuli]

Supersedes: https://github.com/kategengler/ember-cli-code-coverage/pull/103 Text from that PR:

This PR adds support for instrumentation of templates roughly the same as JS instrumentation (using a custom set of helpers and a template AST transform to instrument templates).

There are still some issues with this approach that we need to vet/review.

• Need more testing with "real world" templates.
• Add logic in treeFor to prevent app and addon tree from being emitted unless COVERAGE=true.
• Figure out how to handle instrumentation of usages of htmlbars-inline-precompiler.
• ~Fix issues with istanbul parsing coverage data.~
• ~Decide if we are willing to do a major version bump, or if we have to rewrite in ES5.~

This rendition of the above is mostly the same, but has rebased everything.. I have doubts anything works, so I'll need to comb through the diff of the original PR, and see what I messed up.