Insert commas in urxvt font list

You actually need commas to separate the xft fonts. I have no idea how
this even appeared to work in the past.
Enable italic characters in tmux

Add a new terminfo entry with italics support. It is a modified
screen-256color. Make this new TERM the default in tmux. Update the
zenburn colorscheme to support italics on comments, the same as the GUI
version.