Allow setting of cache directory by `bower_cache` env var

Pre 1.0 the cache directory location could be specified by the `bower_cache` environment variable.  Now that the config option for the cache directory is in `config.storage.packages` it can no longer be set by environment variable (`rc` doesn't support nested vars via environment variables).

This patch allows `bower_cache` to be used to specify the cache directory and resolves #807.

While this works, I'm wondering if it might be better to use the path specified by `bower_cache` as a base path for both `config.storage.packages` and `config.storage.registry` as they're both currently in a `.cache` directory by default.  Thoughts?