add config object that's passed to bower.install

This allows the user to specify offline mode, or any other supported config options defined by bower:
https://github.com/bower/bower/blob/13d37b9d5f971dd298b246bbc2a4233cc2137dd6/lib/config.js

    force: { type: Boolean, shorthand: 'f' },
    offline: { type: Boolean, shorthand: 'o' },
    verbose: { type: Boolean, shorthand: 'V' },
    quiet: { type: Boolean, shorthand: 'q' },
    loglevel: { type: String, shorthand: 'l' },
    json: { type: Boolean, shorthand: 'j' },
    silent: { type: Boolean, shorthand: 's' }

These differ from the options support added for #83 in that they are available to other bower commands than just "install". I'm not really sure of their usefulness, except for offline mode.