Clarify optimizeimg docs; add warnings

The validator will now warn if it detects that a crushed output
is fed into something that is not a crusher.

The is_crusher method of an optimizer shall return True if the
optimisation process is lossless, and does try to find optimal
encoding parameters as opposed to only removing unneeded channels
or reducing palettes.
Fix validator for empty lists (the default value)

Whoops.
Merge pull request #1098 from CounterPillow/optimizehints

Clarify optimizeimg docs; Warn if the order of optimizers does not make sense.