Implement the letters round of the Countdown game

[fpoirotte]

See https://projets.iiens.net/issues/show/363.
Note: maybe this should be a separate module (with a dependency on Erebot_Module_Wordlists).

[fpoirotte]

Will be implemented in a separate module (probably named "Erebot_Module_Scrabble").
Initial idea: the bot chooses 10 letters randomly and displays them on the channel. Whoever proposes the longest word first wins the game. Each game is limited by a 1 minute timer (default limit, but may be set to some other value in the configuration file). Erebot_Module_Wordlists is used to check the words' validity.