Reveal CSS structural update

[ascholerChemeketa]

Removes baked in CSS from reveal template and uses version built by cssbuilder script.

Adds pubvar to specify custom css files for revealjs builds.

[ascholerChemeketa]

Discussion: https://groups.google.com/d/msgid/pretext-dev/CANXmVMC9msE843Zw_eTN5HGUJiFMEGtavh%2BLUs1RSOm_HLr6fw%40mail.gmail.com?utm_medium=email&utm_source=footer

[ascholerChemeketa]

Force pushed to fix typo in commit message.

@rbeezer, @davidaustinm, @sean-fitzpatrick - I view this PR as a self contained (and complete) implementation of what I proposed to do. If other people are actively working on different Reveal related improvements that might intersect CSS, I suggest resolving/merging this before they get too far into things. It will clean up the path for them and give them more tools to work with (custom css file for reveal).

[sean-fitzpatrick]

I don't currently have anything on the go, and don't expect to in the near future. I look forward to seeing how this turns out!

[rbeezer]

Dang. Forgot to put the PR number onto the merge commit.

So here goes, it is at: d6a94f8

[rbeezer]

Thanks for getting this setup! Looks good.

Merged, mostly as-is. Just one hunk of whitespace. ;-)

I've done that flag thing for a derivative stylesheet. Sometimes I've gone back and ripped it out. It'd be nice if we didn't have to do it here. Maybe with an apply-imports though maybe that would mean a template that is not invoked by name. I did not look real close. I did drop a code comment in the RevealJS stylesheet to mildly discourage use.

Would you like to post on -dev to "warn" about CSS changes and to advertise the new option for custom CSS?