There was a situation in which if you you had a lib that depends
on a package with features, whenever you ran the tests for the
package the `rustdoc test` call was failing because rustdoc
was called with the root cfg flags, not the package cfg flags.

This fix solves the issue by keeping track of the cfg flags
per package, so the rustdoc command will be generated with
the correct cfg flags.

In Windows the rustdoc test output sets more double quotes, so
the test doesn't pass. We need to relax the test so it pass in
*NIX and Windows environments.

Travis fails when running `make doc` because of this file.
This should fix the issue.