Remove duplicate code from Customizer/Gruntfile by sharing it

I thought that the "if you update this code here, you'll need to update it there, too" situation is a little bit silly so I outsourced the shared code into Jekyll's `_data` directory.
It's not perfect but I feel it's better than us at some point forgetting about one or the other place.

/cc @cvrebert @XhmikosR 
Use `grunt.file.readJSON`