Add configurable config filenames under :bower

Adopted new nested style for config maps: {:bower {:directory ...}}, except for :bower-dependencies.

Filenames can be set with :bower {:package-file "bower.json", :config-file ".bowerrc"}.

Updated README to reflect bower.json and :bower key changes.
Merge pull request #2 from arbscht/config-filenames-and-bower-key

Add configurable config filenames under :bower