Support custom wrapper component (#9)

Handy for use with [react-transition-group](https://github.com/reactjs/react-transition-group):

```js
<TunnelPlaceholder id="dialog" component={TransitionProvider} />
```

Author: jossmac

src/TunnelPlaceholder.js

@@ -4,10 +4,13 @@ import React, { Component, Fragment } from 'react'
 class TunnelPlaceholder extends Component {
   static propTypes = {
     children: PropTypes.func,
+    component: PropTypes.element,
     id: PropTypes.string.isRequired,
     multiple: PropTypes.bool,
   }
-
+  static defaultProps = {
+    component: Fragment,
+  }
   static contextTypes = {
     tunnelState: PropTypes.object,
   }
@@ -30,7 +33,7 @@ class TunnelPlaceholder extends Component {
 
   render() {
     const { tunnelState } = this.context
-    const { id, children: renderChildren, multiple } = this.props
+    const { id, children: renderChildren, component: Tag, multiple } = this.props
     const tunnelProps = tunnelState.getTunnelProps(id)
 
     if (renderChildren) {
@@ -49,7 +52,7 @@ class TunnelPlaceholder extends Component {
       return null
     }
 
-    return <Fragment>{tunnelProps.children}</Fragment>
+    return <Tag>{tunnelProps.children}</Tag>
   }
 }