Overloaded label-variant mixin for increased flexibility

For now, label-variant can only be called with the background-color parameter, that is darkened for the link hover state. I also wanted to allow the posibility to manually provide the hover color, but as an optional parameter, so I tried this:

I tried
```
.label-variant(@background; @link-hover-color: darken(@background, 10%)) { 
```
But it failed to compile, cause LESS tried to process the background parameter even after the mixin was called.

So I overloaded the mixin, creating one that receives only one parameter, and other that receives both. It could be done in other places as well.

I know is not as easily readable, but I think we can gain a lot of flexibility by doing this.