separates task configs to files

Grunt files tend to grow huge and illegible **very** fast, it would be a real improvement if the task configurations would be separated to files. I chose for a `grunt` directory to add all config files to, but maybe that's not the best choice.

The loadConfig function was copied from https://gist.github.com/thomasboyt/6406507#file-load_config-js, I added a comment as attribution, but I don't know if that's good practice.

Hope you guys accept this one, because the monolithic grunt files drive me crazy :wink: