Add tmux terminfo binary (allows italics in terminal)

Check:
- tmux/tmux#377 (comment)
- https://alexpearce.me/2014/05/italics-in-iterm2-vim-tmux/

It basically boils down to:
```sh
$ cat tmux-256color-italic.terminfo
tmux-256color-italic|tmux with 256 colors and italic,
        sitm=\E[3m, ritm=\E[23m,
	        use=screen-256color,
$ tic tmux-256color-italic.terminfo
```

.tmux.conf

@@ -16,7 +16,7 @@
 # OPTIONS (man tmux)
 # ---------------------------------------------------------------------
 # Improve colors
-set-option -g default-terminal "tmux-256color"
+set-option -g default-terminal "tmux-256color-italic"
 set-option -ga terminal-overrides ",xterm-256color:Tc"
 
 # Set wait time when pressing the escape key to 0 milliseconds.