notmuch-show: add notmuch-show-mark-read-tags option

The `notmuch-show-mark-read-tags' lists tags that are to be applied when
message is read.  By default, the only value is "-unread" which will remove
the unread tag.  Among other uses, this variable can be used to stop
notmuch-show from modifying tags when message is shown (by setting the
variable to an empty list).