Add the possibility of injecting more pip flags

[hmaarrfk]

I think it would be good if we could inject --prefer-binary to pip.

skimage builds are failing because matplotlib 3.0.2 hasn't been uploaded to the wheelhouse.

https://github.com/scikit-image/scikit-image-wheels/issues/13i

[matthew-brett]

Sorry - writing in a rush - but you can override the pip_opts function in config.py to add flags to pip installs.

[hmaarrfk]

and of course, scikit-image already does that!

Thanks!