Plugin idea: const methods, params etc

Hi Noel,

I see you work with clang plugins nowadays. I've got an idea for a
next plugin. I think it would be a good idea to constify things in the
source code, which could improve its readability.
For example member functions which use data members read-only / calls
only const methods / change only mutable data members should have the
const qualifier. I guess it's implementation is something similar to
loplugin::staticmethods.
Similar check can be implemented for parameters, which are not changed
in the function and also for local objects used on read-only way in
the scope.

What do you think?

Thanks,
Tamás