Allow multiple extensions

This does basically two things:

-  Allow to use multiple extensions for source files (deprecate `FLATPAGES_EXTENSION` in favor of `FLATPAGES_EXTENSIONS`)
-  Store file extension in page object

This can be used with renderers that can render more than one markup language (e.g. [pandoc](johnmacfarlane.net/pandoc/)). The renderer can extract the original file extension from the page object in order to figure oout the right source markup.

I am not sure whether it is wise to mix markup languages (or worse: mix file extensions for a single markup, e.g. `.md` and `.markdown`). Still I do not see a reason why it should be impossible.