notmuch-show: add optional argument for query context instead of using global binding...

notmuch-show: add optional argument for query context instead of using global binding notmuch-search-query-string

Also modify the one call to notmuch-show in notmuch.el.  This makes
the call (notmuch-show thread-id) will work when there is no binding
for notmuch-search-query-string; e.g. when called from user code
outside notmuch.