list-group active item text color is incompatible with a transparent background

Created by: freeman

Currently the active list-group-item-text color is defined in relation to the list-group-active-bg color variable (using color: lighten(@list-group-active-bg, 40%);).

In our site we'd like a transparent background color for the background which make the less function fail to evaluate.

I think it would be more logical to define it in relation to the heading color (which is itself defined for the whole list-group-item with @list-group-active-color.).

Maybe using instead color: darken(@list-group-active-color, 10%); ? Alternatively a new variable for this use case would be fine too.

I can submit a pull request if you think this is a good idea.