cli: config: make notmuch_config_open() "is new" parameter input only

cli: config: make notmuch_config_open() "is new" parameter input only

We now have a notmuch_config_is_new() function to query whether a
config was created or not. Change the notmuch_config_open() is_new
parameter into boolean create_new to determine whether the function
should create a new config if one doesn't exist. This reduces the
complexity of the API.