Made italic work for Source Code Pro font

Although Source Code Pro font does not have italic, iTerm2 nightly
build supports fake italic font. After using the following script to add
the xterm-256color-italic, use it in .vimrc

xterm-256color-italic|xterm with 256 colors and italic,
    sitm=\E[3m, ritm=\E[23m,
    use=xterm-256color,

Install:

  tic xterm-256color-italic.terminfo

Usage:

  export TERM=xterm-256color-italic (not needed for vim)