Add BBM.fn.find(); Modify subtree iteration.

; BBM.fn.find(callback, params) -> Array {BBM}
: New method.

  Returns an array of AST nodes the callback returns truthy
  inside the calling subtree, using pre-order traversal.

; BBM.fn.eachPre()
; BBM.fn.eachPost()
: Callback function signature has been changed:

  ; this
  : undefined (Unconfirmed; May be the starting node)

  ; node
  : {BBM} The current node being visited.

  ; params
  : {Anything} Any single extra parameter to pass into
    the callback function.