Added: test coverage with undercover.el

Hi!

I'm author of new library [undercover.el](https://github.com/sviridov/undercover.el) that provides a test coverage check for Emacs packages.

This commit adds coverage check for `f.el` package. [See](https://coveralls.io/files/310505744) my fork coverage results.

It will be awesome if you accept this commit! Thanks!