Allow pinning of package version revisions

[codedmonkey]

Adds the ability to pin a package version to a specific revision which prevents it from updating in the future.

See #8 for more information.

[codedmonkey]

Note that disabling the dirigent.metadata.retain_stale_revisions configuration option will cancel out a pinned revision after the next package update. While this currently is intended behavior, this is probably going to be fixed in a later version.