Support environment markers in setup.cfg.

* d2to1/extern/markerlib/: Bundle marklerlib 0.6.0.

* d2to1/util.py: Check in_cfg_values for environment markers and remove
the value if markers are present and do not match.

* d2to1/zestreleaser.py: Use print function instead of print statement
for py3k compatibility.

* setup.cfg: Package d2to1.extern.markerlib.