feat(divider): added variable for border style

This was hardcoded as 'solid', but deserves to be a variable design decision.

Author: hugopeek

src/definitions/elements/divider.less

@@ -42,8 +42,8 @@
 ---------------*/
 
 .ui.divider:not(.vertical):not(.horizontal) {
-  border-top: @shadowWidth solid @shadowColor;
-  border-bottom: @highlightWidth solid @highlightColor;
+  border-top: @shadowWidth @borderStyle @shadowColor;
+  border-bottom: @highlightWidth @borderStyle @highlightColor;
 }
 
 /*--------------
@@ -117,8 +117,8 @@
     content: '';
     z-index: 3;
 
-    border-left: @shadowWidth solid @shadowColor;
-    border-right: @highlightWidth solid @highlightColor;
+    border-left: @shadowWidth @borderStyle @shadowColor;
+    border-right: @highlightWidth @borderStyle @highlightColor;
 
     width: 0;
     height: @verticalDividerHeight;

src/themes/default/elements/divider.variables

@@ -7,6 +7,7 @@
 --------------------*/
 
 @margin: 1rem 0;
+@borderStyle: solid;
 
 @highlightWidth: 1px;
 @highlightColor: @whiteBorderColor;