Allow clients to override handling of invalid movetext like 'Q1bc2'.

[RogerMarsh]

RogerMarsh/chesstab#15 suggests storing movetext like 'Qb1c2' on it's database as 'Q1bc2'.

A few changes are needed to enable this.

[RogerMarsh]

2143a9b enables this.

[RogerMarsh]

d5f740e abandonned implementation and 8d53bd0 added a simpler but usable alternative.