Add --gem-disable-dependencies flag to allow for excluding one or more dependencies

I've run into situations where i'm building rpm's of gem's to install into an embedded ruby instance (for example sensu).   The embedded ruby may already include one or more gem's that are dependencies of the gem i'm installing.    So in this situation I would like to be able to pass in '--gem-disable-dependencies <gem_name>' and have the specified gem removed from the auto dependency list.