Hacking ggvis add_axis() function to operate as a title function

Version 0.4.3 of ggvis does not have a function to add titles to plots, but still there is a known way to hack the add_axis() function to work as a title function. If a user expects to explore this device many times, it's advised to wrap it into a function. Besides making the code more readable, it's a quicker way to address the problem.