bst artifact checkout should default to directory if neither --tar nor --directory given

Summary

If neither the --directory or --tar options are given for bst artifact checkout, it immediately errors out, saying that one of these options is required:

cd doc/examples/autotools/
bst artifact checkout hello.bst

output:

ERROR: One of --directory or --tar must be provided

Perhaps we should create a default directory to checkout the artifact into if the --directory and --tar options are not given.

This is consistent with how bst workspace open foo.bst behaves.