GraphBase.fromString - add 'eatLabelFn' parameter

This will used by the fromStringLabel functions to consume a label after
the edges given. Labels are described by the regular expression /\d+

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Day 21 Part 4 - Complete Graph.fromStringLabel

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>