…mer)

This allows to define AbstractValues.toString() which will do by default
just printing of all modified (non-default) values, and allows subclasses
to override forEach() to include defaults which will be potentially useful
for other purposes as well.
Meaning, that ValuesToString is now just one possible BiConsumer.

This reduces boilerplate because derived classes don't have to define a
constructor for the AbstractValues subclass.

Rename AbstractOptions.add() to setValue().
Rename AbstractValues.value() to getValueOrDefault().

Also add org.scijava.optional.examples.Playground that has the full
implementation of the example given in the README and package javadoc.