querybts: Add option to directly read mbox using MUA/mutt
querybts supports a -m/--mbox option to dump an mbox directly to stdout, and an --mbox-reader-cmd option to specify a MUA to read an mbox when using the option from the query UI to open the mbox via a mail reader, but does not have any option to directly open the mbox without going through the query UI first.
Add a new option -M/--mua/--mutt, which directly opens the mbox in an MUA (default mutt), exiting when done.
Refactor utils.launch_mbox_reader to separate out URL downloading from reader launching, putting the latter in a new utils.launch_mbox_reader_str.