Fish: Add "home" helper function.

The "home" function cd's to the home directory, and clears the terminal
screen. A stupid helper function.

I've had this for some time, but hadn't committed it. I found several of
these commands with diff ~/src/config/home/.config/fish/functions/
~/.config/fish/functions/. Neat that you can compare directories!
Emacs: Add battery-display binding, C-c p.