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.
This adds a general module infrastructure for configuring email
accounts. The intent is to specify high level information such as IMAP
and SMTP hostnames and login information so that more specific program
and service modules do not have to duplicate options for specifying
accounts.
It is allowed for modules to inject further options within this
namespace where relevant. For example, an MUA may wish add an option
to add per-account filter rules.
Co-authored-by: Matthieu Coudron <mattator@gmail.com>
