NOBUG: Added MoodleList formatter. First cut.

Inspired in the failed/rerun formatters, the idea is to get
a rerun.txt compatible file that later can be splited/balanced
over multiple Behat runners.

It allows to easily get lists of escenarios by using any combination
of tags/names, say:

--dry-run --tags='~@javascript' --format=moodle_list
  (to get all the non-js scenarios and delegate them to a given job)

--dry-run --format=moodle_list
  (to get all the scenarios and split/spread them over a number of runners)

With all the runs using the --rerun facility to be able to "rerun"
failed ones, keeping random failures apart and making it easier to
launch any collection at any moment.