allow `=` in destructuring assignment default

[aeosynth]

https://github.com/satyr/coco/wiki/additions#assignment-defaults-o-----a--b--c

For parameters, = can be used in place of ?.

i would like to use = for destructuring as well

[vendethiel]

Iirc, = in params is only allowed for historical reasons

[aeosynth]

i kinda guessed that - coffeescript uses = for default assignments. i would also be ok with removing = (#207); the inconsistency is what really bugs me