[STUMP] Breaking change to modeline formatting in contrib

[David Bjergaard]

We have the following PR in contrib:

Modified cpu, net and mem modules to export a single modeline formatter.
Each module now comes with its own modeline format string and formatters.
Also added ip address formatter to net module.

Using the single formatter will probably break existing configurations but the default
format string for each module should match the previous behavior.

My thoughts:

1. Do not break init files
2. This is a contrib so breaking is more acceptable.

The first is a hard rule in stumpwm proper. Here I lean towards it.

That being said there is no consistency among the current exported formatters. 

Let me know what you guys think. If I hear nothing I’m going to merge the PR. 

Discussion here: https://github.com/stumpwm/stumpwm-contrib/pull/111#issuecomment-427595398

    David