Added support for screen-it and screen-256color-it (with italics)

The default screen/screen-256color termcap (which tmux uses) doesn't support italics, so programs end up assuming tmux is can't render them despite being capable of doing so. The suggested workaround on https://github.com/ThomasAdam/tmux/blob/master/FAQ is to patch support for italics into the screen/screen-256color termcap, and the suggested name for the new termcap is screen-it/screen-256color-it.

This patch adds support for screen-it and screen-256color-it to vim-fixkey.

Cheers!