Static DAG visualisation with graphviz

[Casyfill]

NOTE: not ready for PR yet, will finalize
NOTE2: my first serious PR, don't judge too quickly

Description

adds luigi.tools.staticviz module, that generates a static (png, pdf, svg) visualization of Luigi Dag
Discussed in #2328

Motivation and Context

In many cases, it is useful to render your task graph as a diagram - for documentation or presentation purposes. staticviz does exactly that - you can render your graphs on the fly (to README.md, for example), apply fancy styles for the presentations - or store it as an svg and hand out to a professional designer.

Have you tested this? If so, how?

• work in progress
• has no interaction with the main body of code

Tasks and Goals

• proof of concept (only supports Python 3)
• fix environments
• make it work on Py 2.7
• add tests
• add support for non-unified styles, add default style (or styles)
• add support for subgraphs, multiple end-points, complex layouts
• beg for PR

[dlstadther]

Unrelated to your code, but why did so many other unnecessary files get committed here? Please clean it up.

I like the idea of this and will review once Py2.7 support is added. Thanks!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If closed, you may revisit when your time allows and reopen! Thank you for your contributions.

[rwagenmaker]

@Casyfill Were there any developments of this feature in a different PR?