Merge: nitweb: show metrics

This PR introduce the metrics displaying in nitweb.

For now, only classes and modules metrics are shown (mclass_metrics.nit, mmodules_metrics.nit, inheritance_metrics.nit).

Some examples (may be long to load on my little server):
* [core::Array](http://nitweb.moz-code.org/doc/core::Array)
* [core::array](http://nitweb.moz-code.org/doc/core::array)
* [core>collection>](http://nitweb.moz-code.org/doc/core%3Ecollection%3E)
* [core](http://nitweb.moz-code.org/doc/core)

Pull-Request: #2219
Reviewed-by: Jean Privat <jean@pryen.org>