Extract $modal-{header,footer}-border-width variables

[skip sauce]
[skip validator]

scss/_modal.scss

@@ -74,7 +74,7 @@
 // Top section of the modal w/ title and dismiss
 .modal-header {
   padding: $modal-title-padding;
-  border-bottom: 1px solid $modal-header-border-color;
+  border-bottom: $modal-header-border-width solid $modal-header-border-color;
   @include clearfix;
 }
 // Close icon
@@ -99,7 +99,7 @@
 .modal-footer {
   padding: $modal-inner-padding;
   text-align: right; // right align buttons
-  border-top: 1px solid $modal-footer-border-color;
+  border-top: $modal-footer-border-width solid $modal-footer-border-color;
   @include clearfix(); // clear it in case folks use .pull-* classes on buttons
 }
 

scss/_variables.scss

@@ -641,6 +641,8 @@ $modal-backdrop-bg:           #000 !default;
 $modal-backdrop-opacity:      .5 !default;
 $modal-header-border-color:   #e5e5e5 !default;
 $modal-footer-border-color:   $modal-header-border-color !default;
+$modal-header-border-width:   $border-width !default;
+$modal-footer-border-width:   $modal-header-border-width !default;
 
 $modal-lg:                    900px !default;
 $modal-md:                    600px !default;