feat: flow-init can use an existing flow object via `flow-object` attr

@AidasK  This is the pull request that you suggested at https://github.com/flowjs/ng-flow/issues/17#issuecomment-50983622.

Since the options could be useless (when flow-object is set) I am not sure whether I should just ignore them or raise some error when we get both options and an existing flow object. For now I ignored them.