src/doc/commands: use filters

[Morriar]

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

  var filter = new ModelFilter(accept_test = false)
  var cmd = new CmdParents(model, main, filter, mentity_name = "MyEntity")

• The string interface through the CommandParser

  [[parents: MyEntity | no-test]]

• The HTTP interface

  /children/MyEntity?no-test=true

[Morriar]

test this please