Set all font-sizes relative to $font-size-base

Created by: tillmon

If all font-sizes are relative to $font-size-base, it would be simpler to change overall font-size:

$font-size-base: 1rem !default;
$font-size-lg:   ($font-size-base * 1.25) !default;
$font-size-sm:   ($font-size-base * 0.875) !default;