Added --- option

--- is equivalent to --, but implies that the argument list for
the command should be reduced to just the mandatory portion.

Resolves #12
Fixes #10 - choose a more benign argument than --ub-select to test passing of arguments.

Now that --ub-print is available
Fixed #11 - Allow tags to be excluded instead of included

You can use `--ub-reject=<tag>` to exclude based on
tags.   Eg running `upbuild --ub-reject=host` would
only run the `make cross` command.  When both select and reject are
specified - tags must both match the selected tag, and not match the
rejected tag.

If both reject and select refer to the same tag, whichever command is
specified *last* will take effect.