Add Global Link Decoration Variable

[ctalkington]

Almost every property in scaffolding allowed for variable override except link text-decoration. This should give a little more control of those properties.

[mdo]

Thanks, but I don't think we need this as a global variable. There will continue to be properties in the future that we consider adding to the BS core and not all of them need to be customizable right out of the gate. The whole variable for every property thing is starting to look like too much overhead honestly, so I'd like to keep us from getting there at all costs.