Add support for css and data attributes that correctly declare and generate inputs and outputs. Support Pandoc --standalone and --self-contained

[deedf]

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This is based on https://github.com/ProdriveTechnologies/bazel-pandoc/pull/13 but fixes the issues pending there, it also adds a "data" attribute that can be used to declare stuff like images and other media referenced in the doc, and have them copied over as runfiles,

[deedf]

Hey thanks for looking at this, I had kind of given up on getting traction here and forgot to update this PR with my later fixes. It is updated now.

[deedf]

It is an almost complete rewrite of the CSS support with added support for the pandoc standalone and --self-contained flags.

[dmadisetti]

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