Many changes

Many changes

* Support for `=` aggregator. #23

* BUGFIX: when --plan is specified (sorry :-X)

* tweak to error handling. when an item's aggregator fails we set it's value to
  $error instead of it's last "ok" value.

  the repl also show this item as "changed"

* tweaks to doctest runnner

* attempt at contains/2 for list membership -- will eventually change to infix
  operator `in`

* cons and nil can't be assigned values.