Allow element type of wrapper to be configurable

## Problem
To use this module within svg, we need to be able to use `<g>` instead of `<div>` for wrappers. 

## Expected
Make <div> configurable at https://github.com/digidem/react-dimensions/blob/master/index.jsx#L170 to other DOM element types

## Code in question
```
 return (
          <div style={wrapperStyle} ref='wrapper'>
```