Add $list-group-item-padding-{x,y} variables

[skip sauce]
[skip validator]

scss/_list-group.scss

@@ -16,7 +16,7 @@
 .list-group-item {
   position: relative;
   display: block;
-  padding: .75rem 1.25rem;
+  padding: $list-group-item-padding-y $list-group-item-padding-x;
   // Place the border on the list items and negative margin up for better styling
   margin-bottom: -$list-group-border-width;
   background-color: $list-group-bg;

scss/_variables.scss

@@ -726,6 +726,9 @@ $list-group-link-color:         #555 !default;
 $list-group-link-hover-color:   $list-group-link-color !default;
 $list-group-link-heading-color: #333 !default;
 
+$list-group-item-padding-x:     1.25rem !default;
+$list-group-item-padding-y:     .75rem !default;
+
 
 // Image thumbnails