Change default ports

[dohseven]

The default ports used by the different Overview plugins are in the range 3000-3100. As these ports might already be used by another application on a server, it should be possible to change it easily, e.g. through the configuration file.

[micahstubbs]

this prevented me from running overiew-local on the first try, as I already had another local server running on port 3000. 💯 for this feature request 😄