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.

renderer.go

@@ -2,10 +2,26 @@ package tea
 
 // renderer is the interface for Bubble Tea renderers.
 type renderer interface {
+	// Start the renderer.
 	start()
+
+	// Stop the renderer, but render the final frame in the buffer, if any.
 	stop()
+
+	// Stop the renderer without doing any final rendering.
+	kill()
+
+	// Write a frame to the renderer. The renderer can write this data to ouput
+	// at its discretion.
 	write(string)
+
+	// Request a full re-render.
 	repaint()
+
+	// Whether or not the alternate screen buffer is enabled.
 	altScreen() bool
+
+	// Record internally that the alternate screen buffer is enabled. This does
+	// should not actually toggle the alternate screen buffer.
 	setAltScreen(bool)
 }