Load ".bower.json" configuration file

Bower generates ".bower.json" files, by basically adding metadata
to regular bower.json files.
But Bower seems to generate those files even if bower.json, package.json
nor component.json files exist. This could fix many cases from #9
and also rely on Bower's ability to figure out clever defaults.
Merge pull request #11 from bclozel/master

Load ".bower.json" configuration file