Better link typography

**Links: This Branch vs. Default**

![new link underline, old link underline](http://tyler.zone/bootstrap-better-links.gif)

There has been a lot of discussion about making more visually appealing links in various places across the web:

1. https://medium.com/designing-medium/crafting-link-underlines-on-medium-7c03a9274f9
2. http://osvaldas.info/keeping-css-short-with-currentcolor

This creates a mixin `.text-decoration` and uses the mixin in `scaffolding.less` for the default `:active` & `:hover` states.

Should fallback to `text-decoration: underline` for browsers [not supporting css gradients](http://caniuse.com/#feat=css-gradients)

May be more trouble than it's worth to implement and maintain—maybe not for upstream :worried: