allow access to user_ns in prompt_manager

[minrk]

adds UserNSFormatter for falling back on shell.user_ns / builtins for name resolution.

[fperez]

I'm thinking rather than making UserNSFormatter take a shell object, instead we should have something like a MultiDictFormatter, whose constructor takes one or more dicts:

def __init__(self, *namespaces):
  self.namespaces = namespaces

...
for container in self.namespaces:
  etc...

then you simply initialize this guy with

self._formatter = MultiDictFormatter(shell.user_ns, __builtins__)

that way the object is generic and has less direct coupling to the specifics of our shell.

[minrk]

The only reason I didn't do that was that I wasn't 100% sure that shell.user_ns is a persistent object across %reset and the like.

[minrk]

Of course, if I instantiate the Formatter inside _render, then that wouldn't be an issue. It would just result in more instantiations, but I'm not sure that's more expensive than "string".format() anyway.