Add support for RGBA OSC response.

[hramrach]

The alpha component is ignored. This merely avoids errors and garbage in the terminal when rgba value is returned in response to a query.

[hramrach]

sh-5.2$ go run examples/hello-world/main.go 

	bold faint italic underline crossout
	red green yellow blue magenta cyan gray
	red green yellow blue magenta cyan gray


	Has foreground color #729fcf
	Has background color #0f0f1f
	Has dark background? true

	"Hello, world!" copied to clipboard

	Triggered a notification
	This is a link
sh-5.2$ 

before:

sh-5.2$ go run examples/hello-world/main.go 

	bold faint italic underline crossout
	red green yellow blue magenta cyan gray
	red green yellow blue magenta cyan gray


	Has foreground color #729fcf
	Has background color #000000
	Has dark background? true

	"Hello, world!" copied to clipboard

	Triggered a notification
	This is a link
^[]11;rgba:0fff/0fff/1fff/f332^[\^[[60;1Rsh-5.2$ ;1R11;rgba:0fff/0fff/1fff/f332;1R

[hramrach]

echo $TERM
rxvt-unicode