Merge: src/doc/commands: use filters

This PR allows the DocCommands to use ModelFilter.

With this, one can setup the filter used by a command from:
* The commands programmatic Nit interface
   ~~~nit
   var filter = new ModelFilter(accept_test = false)
   var cmd = new CmdParents(model, main, filter, mentity_name = "MyEntity")
   ~~~
* The string interface through the CommandParser
   ~~~raw
   [[parents: MyEntity | no-test]]
   ~~~
* The HTTP interface
   ~~~raw
   /children/MyEntity?no-test=true
   ~~~

Pull-Request: #2722
Reviewed-by: Jean Privat <jean@pryen.org>