Add option to pin packages considered unsafe in pip-compile

[alekzvik]

Due to some specifics I had to include some version of pip in dependencies in several projects.
I am not sure it is needed for everyone, so I added a flag to have an option to enable that.

[nvie]

Can you remove the short option (-u) and rename this option (both internally and externally) to --allow-unsafe? Otherwise, looks good to me. Thanks!

[nvie]

Thanks!

[alekzvik]

Thank you for a great project.

@nvie, When do you plan to release this on PyPI?

[nvie]

I might roll a new version some evening this week.

[borgstrom]

@nvie Is there anything else holding up 1.7.1 from landing in PyPI with the addition of --allow-unsafe? I too have run into a situation where I want to include pip.