change node size to fit labels

[stevengj]

It would be nice if the node size automatically stretched out to fit the label text. (Also, currently you can only change the node radius, but for this you would want to stretch the node elliptically in one direction.)

For example, consider the following labelled graph:

and compare to the output of GraphViz for similar labels:

(Note also that GraphViz has proper arrowheads.)

[hdavid16]

How about just moving the label to outside the node? This can currently be done: #161