Add customisable reporterOutputMessage for when report done.

Let the user customise the message to be printed when the report is done. This is for example useful for hooking into CI report importers such as [Teamcity's](http://confluence.jetbrains.com/display/TCD8/Build+Script+Interaction+with+TeamCity#BuildScriptInteractionwithTeamCity-ImportingXMLReports)

I'm using it like so:

```javascript
jshint: {
  teamcity: {    
    options: {
      reporter: 'checkstyle',
      reporterOutput: '.jshint.checkstyle.xml',
      reporterOutputMessage: "##teamcity[importData type='<%= jshint.teamcity.options.reporter %>' path='<%= jshint.teamcity.options.reporterOutput %>']"
    }
  }
}
```

I wasn't sure how to write a test for this change since it's not related to the jshint lib integration which is the only one being tested as far as I can see.

AFAIK without forcing it's impossible to print a statement after a certain task fails, therefore we need this integration IMHO.