Pat's R Markdown helper functions

[kelly-sovacool]

• Created functions in R/rmd_helpers.R:
  • paste_oxford_list() takes a list or vector and returns a string with elements listed in prose using an Oxford comma. (Resolves #20)
  • inline_hook() for knitr to format numbers in a human-readable way. (Resolves #19)
  • set_knitr_opts() to set knitr::opts_chunk and knit_hooks to Pat's defaults. (Resolves #18)
• Documented how to use them in the introductory vignette.
• Wrote a few unit tests. Not exhaustive but probably don't need to be.

[kelly-sovacool]

@pschloss let me know if you have a better way to do it!