Convert local image in markdown file to ReactDom.render

Example:

<img src="./office.jpg" width="100px"> 

to

```__react
ReactDOM.render(<img src={require('./office.jpg')} width="100px"/>, mountNode);```

Usage

Install:

npm i --save bisheng-plugin-local-img

Add 'bisheng-plugin-localimg' to bisehng.config.js's plugins.

module.exports = {
  plugins: ['bisheng-plugin-local-img'],
};