title: How to install pre-release python packages in setup.py install_requires section?
body: <p>In case a python package has a dependency to external libraries we can do:</p>
  
  <pre><code>from setuptools import setup
  setup(name='funniest',
        version='0.1',
        description='The funniest joke in the world',
        url='http://github.com/storborg/funniest',
        author='Flying Circus',
        author_email='flyingcircus@example.com',
        license='MIT',
        packages=['funniest'],
        install_requires=[
            'markdown',
        ],
        zip_safe=False)
  </code></pre>
  
  <p>If I want to install pre-release version of a package in <code>install_requires</code> what should I do? I could not find any flag or command in <code>setuptools</code>, that could do so?</p>
  
  <p>What is the solution for installing unstable pre-release version of packages in <code>install_requires</code>?</p>