Command line option for alpha releases

[Bengt]

pur currently updates gevent to 1.5a4:

https://pypi.org/project/gevent/1.5a4/

I would like to avoid installing such releases that are not considered stable. Could a command line option be added to prevent or allow such alpha releases?

[alanhamlett]

Sounds like a useful feature, would you be able to submit a pull request?

[alanhamlett]

Also, we should check to make sure this isn't already supported with version specifiers in your requirements.txt file.