Add a spine_pre_jacket_html action to output pre #jacket HTML

The specific use case is for adding a sitewide color bar at the top
of the DOM. This should be used with care, but allows custom elements
to be inserted in the DOM above #jacket without having to overwrite
the full header template—something that can lead to long term
maintenance issues
Merge pull request #41 from washingtonstateuniversity/html-actions

[PR] Add a spine_pre_jacket_html action to output pre #jacket HTML