emacs: make message indentation width customisable

emacs: make message indentation width customisable

Till now Emacs UI indents messages according to their respecive
depth of neting in the thread.  The actual width of indentation
per level is hardcoded to `1' space.
This patch makes message indentation customisable by introducing
a variable `notmuch-indent-messages-width' which defaults to `1',
which is the same as before.  Felix could set this variable to
`0' in order to disable indentation, I tested it with a value of
`4' for a clearer separation of messages in a thread.