Add heading font family variable

less/type.less

@@ -54,6 +54,7 @@ a.text-success:focus { color: darken(@state-success-text, 10%); }
 
 h1, h2, h3, h4, h5, h6,
 .h1, .h2, .h3, .h4, .h5, .h6 {
+  font-family: @headings-font-family;
   font-weight: @headings-font-weight;
   line-height: @headings-line-height;
   small {

less/variables.less

@@ -53,6 +53,7 @@
 @line-height-base:        1.428571429; // 20/14
 @line-height-computed:    floor(@font-size-base * @line-height-base); // ~20px
 
+@headings-font-family:    @font-family-sans-serif;
 @headings-font-weight:    500;
 @headings-line-height:    1.1;