A channel is a relationship between 2 directories/boxes/mailboxes
between the local machine (slave) and the remote mail server (master).
Each channel must be given at least:
* an account-unique name
* a pattern for which mailboxes to sync from master
* a pattern for what directory where that mail ends up on the
slave
Additional options can be added later.
To allow supporting more advanced configurations. The local refers to
the "maildir store" configuration, remote to the "IMAP store", and
"channel" to the channel.
This reworks the way program specific email account options are
specified. In particular, we no longer use the deprecated `options`
field of `mkOption`. Instead submodules are used.
