Include PluginError properties in toString

Example output:

```
Error in plugin 'test'
something broke
Details:
    fileName: original.js
    lineNumber: 35
```

Can be turned off by including `{showProperties: false}` in options.

This is piggybacking off of #57
`this.showProperties = (typeof options.showProperties === 'boolean') ? : options.showProperties || false;`

could also use lodash.defaults to clean this logic up