Make excerpts optional in header overlay
Some posts and some header images don't lend themselves well to
displaying the excerpt in the overlay. Make this optional by
introducting a new boolean variable:
`page.header.show_overlay_excerpt`
Set it to implicitly default to `true` so existing users are unaffected by
testing explicitly for "!= false".
This can be enabled globally for a site by changing the default to `false`
in the local _config.yml, or per page by specifying the value in front
matter.
Document show_overlay_excerpt variable
* Add show_overlay_excerpt variable to the Header Overlay field documentation table.
Close#1429
7bf2b58ede