So this allows people to alter the APIs for their plugins and allows
plugins to verify the API is compatible with the one they expect.
It adds a version property to Plugin classes, and has a SemVer checking
method which can be called from other plugins to verify a plugin against
a particular version.
This should have no impact functionality-wise.
The statistics plugin was left out a) as an example and b) because it
was committed by mistake in the first place and was never meant to be a
real plugin.
Mike Auty