fix(dropdown-item-button): Add support for active state (#2212)

src/components/dropdown/README.md

@@ -15,7 +15,7 @@ clicking (or pressing space or enter when focused), not by hovering; this is an
     <b-dropdown-item>Second Action</b-dropdown-item>
     <b-dropdown-item>Third Action</b-dropdown-item>
     <b-dropdown-divider></b-dropdown-divider>
-    <b-dropdown-item>Something else here...</b-dropdown-item>
+    <b-dropdown-item active>Active action</b-dropdown-item>
     <b-dropdown-item disabled>Disabled action</b-dropdown-item>
   </b-dropdown>
 </div>
@@ -58,7 +58,7 @@ Disabled the dropdown item button by setting the `disabled` prop.
 <div>
   <b-dropdown id="ddown-buttons" text="Dropdown using buttons as menu items" class="m-2">
     <b-dropdown-item-button>I'm a button</b-dropdown-item-button>
-    <b-dropdown-item-button>I'm also a button</b-dropdown-item-button>
+    <b-dropdown-item-button active>I'm a active button</b-dropdown-item-button>
     <b-dropdown-item-button disabled>I'm a button, but disabled!</b-dropdown-item-button>
     <b-dropdown-item-button>I don't look like a button, but I am!</b-dropdown-item-button>
   </b-dropdown>

src/components/dropdown/dropdown-item-button.js

@@ -1,6 +1,14 @@
 import { mergeData } from 'vue-functional-data-merge'
 
 export const props = {
+  active: {
+    type: Boolean,
+    default: false
+  },
+  activeClass: {
+    type: String,
+    default: 'active'
+  },
   disabled: {
     type: Boolean,
     default: false
@@ -16,6 +24,7 @@ export default {
       mergeData(data, {
         props,
         staticClass: 'dropdown-item',
+        class: { [props.activeClass]: props.active },
         attrs: { role: 'menuitem', type: 'button', disabled: props.disabled },
         on: {
           click (e) {