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
Output:

```
Error in plugin 'test'
Message:
    something broke
Details:
    fileName: original.js
Error: something broke
    at Context.<anonymous> (/Users/smithm/dev/gulp-util/test/PluginError.js:141:19)
    at callFn (/usr/local/lib/node_modules/mocha/lib/runnable.js:223:21)
    at Test.Runnable.run (/usr/local/lib/node_modules/mocha/lib/runnable.js:216:7)
    at Runner.runTest (/usr/local/lib/node_modules/mocha/lib/runner.js:373:10)
    at /usr/local/lib/node_modules/mocha/lib/runner.js:451:12
    at next (/usr/local/lib/node_modules/mocha/lib/runner.js:298:14)
    at /usr/local/lib/node_modules/mocha/lib/runner.js:308:7
    at next (/usr/local/lib/node_modules/mocha/lib/runner.js:246:23)
    at Object._onImmediate (/usr/local/lib/node_modules/mocha/lib/runner.js:275:5)
    at processImmediate [as _immediateCallback] (timers.js:336:15)
```