Use Heritrix actions to amend running crawls

[machawk1]

See https://github.com/internetarchive/heritrix3/wiki/Action-Directory

For example, to add additional seeds.

Pending confirmation from @anjackson (via IIPC Slack) whether Heritrix 3.2.0 supports it or if #345 is a prerequisite.

Yes, for all of H3's history as far as I can tell. - @anjackson