Remove self from cheetah lookup

[asottile]

Right now self is in the search list. Should change code to explicitly lookup from $self.bar() instead of $bar() potentially meaning ns['bar']() or self.bar()

[asottile]

I think the way this will roll out is:

1. Introduce new lookup functions which will be used when auto self is disabled
2. Introduce an alllow_auto_self compiler setting which does the current behaviour (on by default)
3. Default it to False add a hook for complain()ing when it happens
4. Remove support for the "feature "

[asottile]

Via #247