add iframe component

docs/src/Frame.js

@@ -0,0 +1,29 @@
+import React from 'react'
+import ReactDOM from 'react-dom'
+import {BorderBox} from '@primer/components'
+
+export default class Frame extends React.Component {
+  componentDidMount() {
+    this.doc = this.node.contentDocument
+    this.forceUpdate()
+  }
+
+  render() {
+    const {children, head: _head, ...rest} = this.props
+    const head = this.doc ? ReactDOM.createPortal(_head, this.doc.head) : null
+    const body = this.doc ? ReactDOM.createPortal(children, this.doc.body) : null
+    return (
+      <BorderBox as="iframe" {...rest} ref={node => (this.node = node)}>
+        {head}
+        {body}
+      </BorderBox>
+    )
+  }
+}
+
+Frame.defaultProps = {
+  bg: 'white',
+  border: 0,
+  p: 3,
+  width: '100%'
+}