VIM: Use notmuch CLI for config

This patch switches from reading .notmuch-config directly to using
the CLI the same way that emacs does it.  It actually uses less code
and is probably less error prone.

    Ian
VIM: Make starting in 'insert' mode for compose optional

This adds a variable to make starting in insert mode optional when
composing and replying to emails.  I found it unusual to be started in
insert mode so I thought I'd make it optional as others may find this as
well.

    Ian