Fix 'rebar generate' regression (#253)

If the dir we're about to process contains "reltool.config" and the
command to be applied is "generate", then it's safe to process. We
do this to retain the behavior of specifying {sub_dirs, ["rel"]}
and have "rebar generate" pick up rel/reltool.config. Without this
workaround you'd have to run "rebar -r generate" (which you don't
want to do if you have deps or other sub_dirs) or
"cd rel && rebar generate".