Make tabs stack by default on phones

Created by: ds125v

I'm currently tweaking Bootstrap 2.3 in my app so that tabs ("nav nav-tabs") switch to being stacked tabs ("nav nav-tabs nav-stacked") at phone sizes (<480px), because it looks much better that way if your tabs are long enough to line wrap.

Since Bootstrap 3 is "mobile first", I thought that you might want to consider that as the default behavior, with an opt-out class for those that want their tabs to remain in either of the two states permanently (for example if you know your tab content ahead of time is going to be very narrow, just icons perhaps).

Similar may apply to other related navs, I've used them less so I don't have a firm opinion.