added setOption public method to support dynamic min/max demo

Although this repo doesn't currently contain a dynamic min/max demo page, the http://flexslider.woothemes.com/ site still does include that example, and it's broken (watch the console for JS errors when resizing the window, the page tries to modify flexslider.vars, which doesn't exist — vars is not a public property on the plugin instance).

This pull request:
- adds a simple public setOption method to flexsider
- uses that method to to reintroduce the dynamic min/max demo and get the demo working
- updates the nav on all the demo pages to include the dynamic min/max demo
- fixes minor spelling/formatting bugs