Add option to disable carets on dropdowns through $enable-caret

scss/_dropdown.scss

@@ -6,21 +6,7 @@
 
 .dropdown-toggle {
   // Generate the caret automatically
-  &::after {
-    display: inline-block;
-    width: 0;
-    height: 0;
-    margin-left: $caret-width * .85;
-    vertical-align: $caret-width * .85;
-    content: "";
-    border-top: $caret-width solid;
-    border-right: $caret-width solid transparent;
-    border-left: $caret-width solid transparent;
-  }
-
-  &:empty::after {
-    margin-left: 0;
-  }
+  @include caret;
 }
 
 // Allow for dropdowns to go bottom up (aka, dropup-menu)
@@ -32,10 +18,7 @@
   }
 
   .dropdown-toggle {
-    &::after {
-      border-top: 0;
-      border-bottom: $caret-width solid;
-    }
+    @include caret(up);
   }
 }