✨ Add dynamic styles and css classes as prop

You can now pass in css classes as a string for the surrounding div of the canvas and a styles object which will be applied as inline styles.
This way you can have a dynamic height with `reposnive: true`

## Example

```js
<template>
   <line-chart :styles="myStyles"/>
</template>

<script>
export default {
  data () {
    return {
          height: 100
    }
  },
  computed: {
    myStyles () {
      return {
        height: `${this.height}px`,
        position: 'relative'
      }
    }
  }
}
</script>
```

src/BaseCharts/Pie.js

@@ -5,7 +5,10 @@ import { mergeOptions } from '../helpers/options'
 export default Vue.extend({
   render: function (createElement) {
     return createElement(
-      'div',
+      'div', {
+        style: this.styles,
+        class: this.cssClasses
+      },
       [
         createElement(
           'canvas', {
@@ -33,6 +36,20 @@ export default Vue.extend({
     height: {
       default: 400,
       type: Number
+    },
+    cssClasses: {
+      type: String,
+      default: ''
+    },
+    styles: {
+      type: Object,
+      default () {
+        return {
+          width: '100%',
+          height: '200%',
+          position: 'relative'
+        }
+      }
     }
   },