title: Can Pip install dependencies not specified in setup.py at install time?
body: <p>I'd like pip to install a dependency that I have on GitHub when the user issues the command to install the original software, also from source on GitHub. Neither of these packages are on PyPi (and never will be).</p>
  
  <p>The user issues the command:</p>
  
  <pre><code>pip -e git+https://github.com/Lewisham/cvsanaly@develop#egg=cvsanaly
  </code></pre>
  
  <p>This repo has a <code>requirements.txt</code> file, with another dependency on GitHub:</p>
  
  <pre><code>-e git+https://github.com/Lewisham/repositoryhandler#egg=repositoryhandler
  </code></pre>
  
  <p>What I'd like is a <em>single command</em> that a user can issue to install the original package, have pip find the requirements file, then install the dependency too.</p>