Allow zc.buildout-like interpolation.

[pmav99]

In Python 3, the configparser found in the stdlib's configparser has the so called "ExtendedInterpolation" which supports the syntax used by zc.buildout.

I think it would be a nice addition to have this functionality in configobj too.

[robdennis]

That is a pretty neat piece of functionality, though from what I've seen of the interpolation code, I'm not confident it's something that can be added on and would need a rewrite.