Adding support for non-default port

[hobbeswalsh]

Hi,

We have multiple instances of MongoDB running on multiple ports on our site, so I thought I'd modify this code and send it along your way for review.

Thanks, Robin

[hobbeswalsh]

Actually, I'm sorry, I don't think this is munin-compatible. Let me re-write it.

[tommedema]

Can this be implemented, please? Also could you add build instructions or make clear how to set the correct port to the readme?