Add usemin option

From @pheuter:

@ericclemmons In my case, I am generating two javascript files, vendor.js and application.js. When concatenating ng-templates, I need to append solely to application.js. The error I was running into was due to the templates file being concatenated to both files and that caused errors.

// build:js(.) javascripts/vendor.js

// build:js(.) javascripts/application.js

In which case, the jump to 0.5.0 may not be warranted (whoops!), because this solution can let someone specify usemin: 'javascripts/application.js' to target that specific dest.

concat is useful if you have a specific concat task, but trying to target what usemin generates for concat is annoying at best.

[orlando]

I’ll leave this here in case someone is looking into how make grunt-rev work with grunt-angular-templates

https://github.com/yeoman/grunt-usemin/issues/235#issuecomment-33266949

[marcalj]

@ericclemmons Works perfectly! Excellent job! 😃