Fix a race where artifacts could print when exiting a program

This commit also consolidates the exit operations for consistency's
sake, and adds a kill() method to renderers for stopping them without
performing any final rendering.

standard_renderer.go

@@ -61,13 +61,19 @@ func (r *standardRenderer) start() {
 	go r.listen()
 }
 
-// stop permanently halts the renderer.
+// stop permanently halts the renderer, rendering the final frame.
 func (r *standardRenderer) stop() {
 	r.flush()
 	clearLine(r.out)
 	close(r.done)
 }
 
+// kill halts the renderer. The final frame will not be rendered.
+func (r *standardRenderer) kill() {
+	clearLine(r.out)
+	close(r.done)
+}
+
 // listen waits for ticks on the ticker, or a signal to stop the renderer.
 func (r *standardRenderer) listen() {
 	for {