Adds button styles to `.list-group-item`

Includes buttons with anchor sytles and modifies buttons to match width
and text alignment.

less/list-group.less

@@ -38,12 +38,13 @@
 }
 
 
-// Linked list items
+// Interactive list items
 //
-// Use anchor elements instead of `li`s or `div`s to create linked list items.
+// Use anchor or button elements instead of `li`s or `div`s to create interactive items.
 // Includes an extra `.active` modifier class for showing selected items.
 
-a.list-group-item {
+a.list-group-item,
+button.list-group-item {
   color: @list-group-link-color;
 
   .list-group-item-heading {
@@ -59,6 +60,11 @@ a.list-group-item {
   }
 }
 
+button.list-group-item {
+  width: 100%;
+  text-align: left;
+}
+
 .list-group-item {
   // Disabled state
   &.disabled,

less/mixins/list-group.less

@@ -5,7 +5,8 @@
     color: @color;
     background-color: @background;
 
-    a& {
+    a&,
+    button& {
       color: @color;
 
       .list-group-item-heading {