use tput to test for italics support on POSIX systems (fixes #311)

[AprilArcus]

[delphinus]

system() call spends time (a bit) and has a negative effect to launching time for Vim. I prefer to using an option to preventl system() call. Such as

if has("gui_running") || g:solarized_italic_in_terminal || ( has("unix") && system("tput sitm") == "\033[3m" )

[kanreki]

Is anything happening with this PR? My terminal can display italics just fine, but it doesn't set a TERM_PROGRAM environment variable (and I'm reluctant to fake it, as I don't know what other side effects that would have).

Is there any clean way to enable italics, short of patching an ugly hack into (my own copy of) colors/solarized.vim?