bump version and update history
Merge branch 'pchaigno-rmarkdown' into release-1.3.0

* pchaigno-rmarkdown:
  Case-insensitive on just markdown
  Perform case-insensitive test for file extensions
  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
  Render RMarkdown (.rmd) as Markdown
  Force encoding of posix_spawn output
  Force encoding of posix_spawn output
  Fix a spelling mistake
  rst2html: Extend the field_name_limit to 50
Merge remote-tracking branch 'origin/master' into release-1.3.0

* origin/master:
Update Changelog for 1.3.0
Merge pull request #318 from github/release-1.3.0

Release 1.3.0