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/components.md

@@ -0,0 +1,94 @@
+Vue Select utilizes child components throughout, and exposes an API to overwrite these components 
+with your own, using the `components` `{Object}` prop. When implementing the `components` prop in 
+your code, Vue Select merge it's default components with any keys that you set in the object.
+
+Your object will be merged with the object that is exported below:
+                   
+<<< @/src/components/childComponents.js{4-7}
+
+You can override the value of any of these keys with your own components.
+
+## Available Components
+
+### Deselect
+
+You may wish to roll your own deselect button. `Deselect` is used within tags on 
+`multiple` selects, and serves as the clear button for single selects. Maybe you just want to use
+a simple `<button>Clear</button>` instead.
+
+```html
+<v-select :components="{Deselect}" />
+``` 
+
+```js
+computed: {
+  Deselect() {
+    return Vue.component('Deselect', {
+      render (createElement) {
+        return createElement('button', 'Clear')
+      }
+    })
+  }
+}
+```
+
+<ClearButtonOverride />
+
+The same approach applies for `multiple` selects:
+
+<MultipleClearButtonOverride />
+
+### OpenIndicator
+
+The `OpenIndicator` component is the 'caret' used within the component that adjusts orientation
+based on whether the dropdown is open or closed.
+
+```html
+<v-select :components="{OpenIndicator}" />
+```
+```js
+computed: {
+  OpenIndicator () {
+    return Vue.component('OpenIndicator', {
+      render (createElement) {
+        return createElement('button', '🤘🏻');
+      },
+    });
+  },
+},
+```
+
+<OpenIndicatorOverride />
+
+## Setting at Registration
+
+If you want to all instances of Vue Select to use your custom components throughout your app, while
+only having to set the implementation once, you can do so when registering Vue Select as a component.
+
+```js
+import Vue from 'vue';
+import vSelect from 'vue-select';
+
+/**
+* Create custom components to override defaults.
+* @type {{OpenIndicator: *, Deselect: *}}
+*/
+const components = {
+  Deselect: Vue.component('Deselect', {
+    render: (createElement) => createElement('button', '❌'),
+  }),
+  OpenIndicator: Vue.component('OpenIndicator', {
+    render: (createElement) => createElement('span', '🔽'),
+  }),
+};
+
+// Set the components prop default to return our fresh components 
+vSelect.props.components.default = () => components;
+
+// Register the component
+Vue.component(vSelect)
+```
+
+<CustomComponentRegistration />
+
+