Support special setuptools directives in setup.cfg

[zsol]

Setuptools' doc specifies a bunch of interesting directives that are used in the wild. They need special interpretation

[thatch]

Indeed. In addition to attr: there is also file: from #4

I've been putting this off until the api change necessary to replace the "??" string with a proper Unknown sentinel, where I would also probably replace the string "attr: foo" with Attr("foo") that has a .get().