Add new_command setting

new_command will get executed when the current mailbox receives new
mail. Two values are formatted into this string, the first is the number
of new messages and the second the number of unread messages.

If this value is not set, no command is executed.

Note that we are using snprintf to avoid a buffer overflow, we add 20
more characters as that would be the maximum number of digits two
integers together could take, plus one more for the NULL character.
@SantiagoTorres and I discussed this offline.