FR: allow disabling individual releaseable units in a group/bulk release config

Hi. Quick feature request about controls for scenarios where a single release config controls multiple releaseable units.

It would be nice to embargo a particularly releasable unit from inclusion in a bulk release.

A simple straw man:

* In google-cloud-go, we have a bulk release with N independent modules.
* Sometimes, a given module is getting manual PRs that modify the behavior and those changes may be over an extended period (e.g. longer than the typical 1/week release cycle).
* We want to release everything BUT the affected module, rather than delaying all the modules from being released.

The issue names no files or tests. Start by tracing how bulk release configurations select releaseable units, then determine where an individual unit can be excluded without affecting the others. Done means a documented configuration behavior, coverage for mixed included and excluded units, and confirmation that other units still release normally.

Written by the indexing model from the issue text.