Add logconfig option to write logs to specified file

[ntrvic]

We are using this plugin at Conductor and would really benefit from this addition. This PR will allow the user to pass the 'file' log configuration option for a given image and specify a location for the logs to be written to. For example:

<log>
    <file>${basedir}/app.out</file>
</log>

If the file option is empty, it will print to System.out, otherwise it will use FileOutputStream and print to the specified location.

[rhuss]

Thanks !

If you don't mind, I'm just polished it a bit:

• No color if logging to a file
• Append to file (user must rotate on its own)
• Add documentation
• Refactoring (the map holding the streams is not needed)
• Add a property docker.logStdout to overwrite any configured file (useful when doing a docker:log)

[rhuss]

Will be available in the next release.