add github-flavored markdown and aligned output for Table

Co-authored-by: Andreas Klöckner <inform@tiker.net>

Doc fixes

See merge request !45

See https://docs.github.com/en/github/writing-on-github/organizing-information-with-tables
for specification

Topological ordering with priorities

- adds link for Kahn's algorithm
- replaces compute_topological_order by compute_topological_order_v2
- introduce class HeapEntry

Add missing codegen.rst to doc directory

Add pytools.codegen

This change takes common code generation functionality out of
pytools.py_codegen and makes it a separate module.

A language-independent parent class of PythonCodeGenerator is also
added.

The version is bumped to 2020.4

Fix typos in warnings

June monster maint

See merge request !44

# Conflicts:
#   setup.py