(1 Apr 2020, @ankostis, BROKEN): ortho plots

(broken: wheel in PyPi is missing sphinxext package)

Should have been a major bump due to breaking rename of Plotter class,
but...no clients yet.

• ENH(plot): plot edges in grapos with Graphviz_ splines=ortho.
• REFACT(plot): rename base class from Plotter --> Plottable;
• enh(build): add [dev] distribution extras as an alias to [all].
  doc: refered to the new name from a new term in glossary.
• enh(site): put RST substitutions in :confval:rst_epilog configuration
  (instead of importing them from README's tails).
• doc(quickstart): exemplify @operation as a decorator.