Add support for specifying tags during "notmuch new"

[David Edmondson]

This patch set allows a user to specify a list of tags to be
added/removed to messages discovered during "notmuch new".

Two use-cases are envisaged:
    1) A chunk of messages was just dumped into the configured
       directory by hand, and the user doesn't want the 'inbox' tag
       applied to them. Run 'notmuch new -inbox'.
    2) A periodic mail processing script wants to add new messages to
       the database, then process those newly added messages to add
       convenience tags, etc. without worrying about the user or other
       instances of the script manipulating tags at the same time. Use
       this approach:
         KEY=$RANDOM
	 notmuch new +$KEY
	 notmuch tag +notmuch tag:$KEY and to:notmuch at notmuchmail.org
	 notmuch tag +gnus tag:$KEY and to:ding at gnus.org
	 ...
	 notmuch tag -$KEY tag:$KEY