Use regex to accept Windows, OS X, and Unix line endings instead
of expecting Unix line feeds.

Resolves https://github.com/getnikola/nikola/issues/832
Documenting branch markdown-for-osx-and-windows-authors
Used solution from 10e6d7b on bbcode, misaka, rest, and textile.
Merge pull request #833 from Aeyoun/markdown-for-osx-and-windows-authors

Compiler plugins expects two Unix line feeds instead of two of any line endings