Use Standard Markdown for files within a repo, not GFM

[NV]

How to reproduce the bug:

1. Open http://github.com/NV/github-live-preview/blob/gh-pages/README.md or another markdown file which you can edit
2. Press "edit" button
3. Type
   first line
   second line
4. Save
5. Result and preview will be different, because "GFM treats newlines in paragraph-like content as real line breaks, which is probably what you intended."