Default no-args behavior sends DM with "Testing!" message to default recipient

[babetoduarte]

Currently TootNotify sends out a DM to the default recipient, when the tootnotify command is executed without any parameters:

INPUT

foo@bar:~$> tootnotify
foo@bar:~$>

RESULT

Instead of this, TootNotify should inform the user that no arguments were provided, and while the DEFAULT_RECIPIENT should populate the recipient's argument -r/--recipient, the message argument -m/--message should be None, and no message should be sent.