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
I added the `\nStack:` for coordination with the properties that get output, since the two would start to run together and it was difficult to distinguish between the two.