add polymode recipe

Hi. 

Everything works fine with my local tests except that autogenerated autoloads file of the installed package is empty. Not sure if it is a feature or a bug. I have no problems with generating autoloads locally with `update-file-autoloads`. 

Thanks.

----
[Polymode](https://github.com/vitoshka/polymode) is an emacs package that offers generic support for multiple major
modes inside a single emacs buffer. It is lightweight, object oriented and
highly extensible. Creating new polymodes commonly takes just several lines of
code.

Polymode also provides extensible facilities for external literate programming
tools such as exporting, weaving and tangling.