Ability to include/extend other inis in tox.ini

Issue #364 on hold
Andy Freeland
created an issue

it would be useful to be able to share/reuse config (for example, a style toxenv) through some sort of include syntax in tox.ini.

e.g. in tox.ini:

[tox]
envlist = py27,py35,style
include = tox-style.ini

and style.ini:

[testenv:style]
....

That's almost definitely not the right syntax to use, but I think it gets the idea.