Add tooltips to DAG visualization about bionic

[lexi-squid-ding]

@jqmp I started looking into this and looks like as of today, Pillow does not support SVG. But it's straightforward to get a SVG from a pydot graph, so we can evaluate additional packages like Cairo and svglib to actually render the image.

Let me know if you have a preference

[jqmp]

Oh, good point. I'm cool with adding a dependency if necessary. That said, it seems to me that we want to support two main use cases:

# Saving to a file.
flow.render_dag(format='svg').save('my_dag.svg')

# Inside a Jupyter notebook, this should render the image.
flow.render_dag(format='svg')

I don't know offhand if those libraries provide a class that supports both of those. However, since Jupyter knows how to render SVGs already, it would probably be easy to define our own SvgImage class that implements save and _repr_svg_ methods. What do you think?

[lexi-squid-ding]

Will look into whether either library does this natively, otherwise adding the new class makes sense to me.