[Feature request] Make "single author" check configurable

[HakanS]

Makes no sense in a single (or few) user environment. The plugin could be useful
for example in a CMS use case, guiding novice users to better pages but they
will disable the plugin if every page has an error they can't remove.

/Håkan

Best usability would be link to config from error message "Yes, there is just me writing"