Added method and option for multiple menus

[juliankrispel]

This has been mentioned in issue #32 and I needed it for my own work.

Cheers!

[jbernal]

Hi juliank - Any progress on having functionality for multiple responsive-nav instances?

[juliankrispel]

Hello @jbernal why don't you ask the plugin author @viljamis ? I can't accept/comment on pull requests in his stead

[arielsalminen]

@juliankrispel This doesn't seem to calculate the height correctly for multiple menus. For me it returns height always as "undefined" if the new option is used, which then breaks the transition.

Another issue with the pull, but not that big one, is that it doesn't follow the guidelines which can be found here: https://github.com/viljamis/responsive-nav.js/blob/master/CONTRIBUTING.md