slide-level in YAML metadata

[raffaelemancuso]

I request the possibility to specify the --slide-level option for slideshows with the YAML metadata for beamer.

[P-N-L]

I think this makes a lot of sense. The slide level might be different for different slide decks, it should be able to be considered a property of each individual document.

[jgm]

Conceptually, slide-level isn't metadata. It's a processing instruction. What I do is create a Makefile so that the instructions needed to build each document are right there with the document; I don't have to remember anything but "type make." Defaults files are another good solution.