Added distDirs config key

Recently trying to use Yeoman for project depending on the latest version of jQuery, I found that jQuery does not have a main file specified in it's bower.json and that the actual main file 'jquery.js' is located within a 'dist' folder.

No wanting to use the bower.json file for fixing this, I added a config option distDirs for standard distribution directory names with default value of ['dist'].

As currently the detection of dependencies was using a check for component-named file in the component directory, I added an additional check for a distDirs-based directory check for the same file.

The distDirs can also contain an object value where each component has it's own specific distDirs array. For this a new test was added.

I saw the override related commit just when I rebased a few minutes ago - it's solving similar problem but in a different way. Since I put some effort into this, I let it up to you to decide if this can be useful.

Cheers