Support .markdown file extension (in addition to current .md)

Currently, "md" is the only extension identifying markdown files for Doxia. There are many [0] extensions identifying markdown. GitHub uses "md", but it is also interpreting markdown somewhat differently [1]. Wikipedia article [2] offers "markdown" as an alternative extension, citing [3] the author of markdown, who agrees [4] with proponents of this extension.

Please, add a support for "markdown" extension;
possibly keeping it for standard markdown and leaving "md" for the GitHub style.

[0] https://gist.github.com/schwa/8311179
[1] https://help.github.com/articles/github-flavored-markdown
[2] https://en.wikipedia.org/wiki/Markdown
[3] http://daringfireball.net/linked/2014/01/08/markdown-extension
[4] http://hiltmon.com/blog/2012/03/07/the-markdown-file-extension/