Suggestions about performance

`ngmin.annotate()` is slow and rewrite all the source using esprima/escodegen (and that is ok). 
But, with large projects, browserify may transform many kind of sources (not only javascript actually) that do not need to be transformed by `ngmin.annotate()`.

May I suggest:

  1. Line 5: Check that source file name is likely javascript
  2. Line 14: Test the source for a `angular.module` statement. This is anyway required by ngmin, and that may exclude most of the not-angular sources.