differentiator: main: actually add the '-Q' option to the getopt() invocation.
differentiator: main: fix mode of argument (optional) of '-S'.
differentiator: README.md: add a README and a rule to build an HTML version of it using pandoc.
differentiator: visitor-simplify: WIP: add product expansion by distributive law of multiplication.

(a + b)(c + d) = ac + ad + bc + bd

This works very, very slow. Don't know why. To be done.