Major rework

* Underscore 1.7.0
* Always use "underscore" to produce the template source.
* Instead of `noMinify`, set `minifierOpts` to false.
* Removed `withImports`
* Store `Buffer` chunks instead of `String` chunks during transform.
* Expose `compile` for direct use.
* Switched from `mocha`/`should` to `tape`.
* Rewrote tests to use `vm` instead of `eval`