Merge: nitdoc: Use wikilinks on README files

Merge: nitdoc: Use wikilinks on README files

This PR introduce the use of wikilinks in README files.

It's mainly a request for comments on what you want, what you need and what you like from Nitdoc and README writting.

0f6ff78 gives an example of README file written using wikilinks. The processed result by nitdoc is visible [here](http://gresil.org/jenkins/job/CI-nitdoc/ws/doc/stdlib/group_github.html).

Supported commands for now:

* `[[doc:MEntity]]`: include the documentation article or an MEntity
* `[[list:MEntity]]`: list classes or methods from than MEntity

Old API pages still available, there are prefixed with `api_`.

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)

Some question still need to be answered:
* What to do with old API page (http://gresil.org/jenkins/job/CI-nitdoc/ws/doc/stdlib/api_group_github.html)
* What comment to display in API page (http://gresil.org/jenkins/job/CI-nitdoc/ws/doc/stdlib/api_group_github.html)
* What commands should be supported by wikilinks directive?

Pull-Request: #1447
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>