Add option to expand wildcard imports

[jph00]

E.g

from foo import *

...would be optionally exported to:

from foo import a,b

...if a and b were used in that module.

There's something a bit similar here we could borrow ideas from: https://github.com/fake-name/autoflake/blob/master/autostar.py

[jph00]

Per discussion with @dleen, also consider a tool which simply reports what symbols are used by an import, rather than replacing them.