A group is a grouping of channels together, so that many channels with
very different names can be handled as a single entity.
Groups are unique in mbsync because they will shadow channels that
have the same name on the command-line.
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.
