Add API for overwriting default components (#850)

* implement API for overwriting child components

* add test coverage

* update documentation for Components & Styling

* update docs

* refactor API, update docs

* remove the service worker

* fix tests

docs/guide/slots.md

@@ -0,0 +1,23 @@
+::: tip 🚧
+This section of the guide is a work in progress! Check back soon for an update.
+Vue Select currently offers quite a few scoped slots, and you can check out the 
+[API Docs for Slots](../api/slots.md) in the meantime while a good guide is put together.
+:::
+
+#### Scoped Slot `option`
+
+vue-select provides the scoped `option` slot in order to create custom dropdown templates.
+
+```html
+<v-select :options="options" label="title">
+    <template slot="option" slot-scope="option">
+        <span :class="option.icon"></span>
+        {{ option.title }}
+    </template>
+  </v-select>
+``` 
+
+Using the `option` slot with `slot-scope="option"` gives the 
+provides the current option variable to the template.
+
+<CodePen url="NXBwYG" height="500"/>