color: support "italic" attribute

color: support "italic" attribute

We already support bold, underline, and similar attributes.
Let's add italic to the mix.  According to the Wikipedia
page on ANSI colors, this attribute is "not widely
supported", but it does seem to work on my xterm.

We don't have to bump the maximum color size because we were
already over-allocating it (but we do adjust the comment
appropriately).

Requested-by: Simon Courtois <scourtois@cubyx.fr>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>