Give navbar-brand it's own color variables

Created by: David-Tennant

Currently the only color variables for .navbar-brand is the same as navbar dark active links. I am not quite sure if this is a feature request or an issue.

My suggestion would be to add color variables for the navbar-brand

Use case : A theme wants to use text instead of a logo as their brand appears on majority of the page already.

Current : _navbar.scc:250

  .navbar-brand {
    color: $navbar-dark-active-color;

    @include hover-focus {
      color: $navbar-dark-active-color;
    }
  }

Could there be a seperate color variable that can be overridden if needed? My current solution below:

_navbar.scc:250-257

  .navbar-brand {
    color: $navbar-brand-color;

    @include hover-focus {
      color: $navbar-brand-hover-color;
    }
  }

_variables.scss:649-651

$navbar-brand-color : $white !default;
$navbar-brand-hover-color : $white !default;
$navbar-brand-font-size:            $font-size-lg !default;
...other brand variables