add grunt-chrome-manifest task to handle manifest

i made a new grunt task[1] to handle manifest.json of chrome. the grunt task has a two feature. one is adds js/css file list in manifest to configuration of cssmin/uglify/concat, like useminPrepare task. and auto increment build number task. 

i'd like to use grunt-chrome-manifest to both generator, chrome-extension and chromeapp.

[1]: https://github.com/ragingwind/grunt-chrome-manifest