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
You know that doesn't look exactly like I would have expected. I expected to have a `Stack:` in there. I'll do some more cleanup tomorrow.