Support all xterm variants with color

[relistan]

I see from the history that how to support color or not has been changed a few times. Originally it supported any TERM var that contained xterm. Then it was looking at TERM for 256. Now it looks for color.

Some modern terminals, e.g. Kitty, have their own terminal type in order to support extended integration between the shell and the terminal. Kitty's default terminal type is xterm-kitty. I could hack around this by renaming it, however, I feel like goconvey should probably support modern terminals like this.

Probably the better strategy at this point is to have an env var that disables color. However, I did not attempt to mess with that here.

I've been using xterm since at least 1992 and AFAIK it has supported color since well before then. goconvey is not using colors beyond the standard range (e.g. not using 256 color range). So I'm proposing here that goconvey also support color on any terminals with a TERM var containing xterm. This should not affect the behavior of anyone with other terminals on which color is currently supported.