#11 expose disable_font_awesome in bulma::bulma_document()

output: 
  bulma::bulma_document:
    toc: true
    pandoc_args: ["--variable", "no-font-awesome"]

Currently, need to do this: ``` output: bulma::bulma_document: toc: true pandoc_args: ["--variable", "no-font-awesome"] ``` which is too much and I always forget. It's also not documented in `bulma_document()`. Would be better to move it to an arg of `bulma_document()`.