Support reading constants from file

Currently the only way you can read constants from file is by using the grunt.file.readJSON utility method. This is fine as long as you configuration file is static and doesn't change during the lifecycle of the Grunt build process. That's not the case for us: we use mustache templates to dynamically generate constants based on external input.

Also, it's much simpler to just specify the file name be agnostic to the way the file is read.