Accept just a bookmark name as a short form of the '-g' option. If just
one arg is supplied, and it doesn't start with a dash, then it is assumed
to be a bookmark name and _goto_bookmark is called. [ bm BOOKMARK_NAME ]
Add a -h option to print usage info with same behavior as supplying no args
Add echoing an error message if an unrecognized option is given, followed
by usage info
Augment usage info to reflect these changes and update README.md
