Merge: nitdoc: refactoring and cleaning

Now that nitx and nitdoc are fully merged with the same structure and services, we can start the refactoring to limit the differences between both tools.

Note that the first four commits are from #1401.

Demos from [Jenkins::CI-nitdoc](http://gresil.org/jenkins/job/CI-nitdoc):
* [Standard library](http://gresil.org/jenkins/job/CI-nitdoc/ws/doc/stdlib/index.html)
* [Nit compilers and tools](http://gresil.org/jenkins/job/CI-nitdoc/ws/doc/nitc/index.html)

Pull-Request: #1416
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>