Instead of being a command line-only option, the string of extra rpmbuild
arguments can now be passed in via the general builder args mechanism, so it
can even be configured from releasers.conf.
Fixes#155
Particularly useful when creating new projects and wanting to avoid
creating an initial tag, this allows `tito build --test --srpm` etc
to run without any tag.
With a general 'scl' builder option, a software collection name can be
specified irrespective of the releaser used. RsyncReleaser's own 'scl'
option has been deprecated in favour of using 'builder.scl' in
releasers.conf.
