0.12 release

Hi everyone, Seems like we are close to clearing the 0.12 milestone: https://github.com/scikit-image/scikit-image/pulls?q=is%3Aopen+is%3Apr+miles... It's been a while since we had a release and lots of great stuff has been merged in the mean time. I suggest to freeze any other PRs until we release 0.12. Maybe we can aim to release 0.12 in the next days? Cheers, Johannes