[WIP] upgrade to shower 2.7.0

[maxheld83]

I thought I'd be nice to upgrade to the latest upstream shower, which offers some interesting new features.
Also makes it easier for users if the upstream docs are in sync with this package.

This is still work in progress and I might have broken things.
I'll use this in my own work and try to test / fix all the features.
Thought I'd raise this PR already to let people know that this is being worked on.

Will update this conversation once I think it's ready for review.

I'm only using the built-in theme and this probably breaks compatibility with the custom themes.

TODOS

• pass on progress bar arg
• pass on aspect ratio
• pass on color_key
• fix code highlighting
• fix shout color
• fix multiple heading per slide
• (maybe?) compatibility with theme pkgs (such as earl, mango, etc.)
• set up automatic CI for skeleton
• enable remaining pandoc features
• maybe source out the pandoc template to a separate repo
• enable skeleton for RStudio again (deleted because of code duplication; example currently just lives in, well, example)