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
@mtscout6 What does the final output of a toString() look like in both cases on the PluginError? I'm assuming it has changed with this PR. A screenshot would be handy