Render RMarkdown (.rmd) as Markdown
Perform case-insensitive test for file extensions
Merge remote-tracking branch 'origin/master' into pchaigno-rmarkdown

* origin/master:
  Escape the filepath to rest2html so paths containing spaces will be passed to CommandImplementation#execute correctly.
  Update README.md
  Tables are cooler
  Organize the README's whitelisted elements a bit better
  Force encoding of posix_spawn output
  Force encoding of posix_spawn output
  Update WikiCloth to 0.8.1 and support footnotes
  rst2html: Extend the field_name_limit to 50
Case-insensitive on just markdown
Merge pull request #358 from github/pchaigno-rmarkdown

Use case-insensitive regex for file extension matching.