title: Centralized requirements file
body: <p>Is there a way to consolidate, or better yet, create a hierarchical requirements file in a given repo to be used across multiple virtual environments...perhaps in yaml format, so I can handle version exceptions for specific projects? I have found that when I am working on various project environments with their own <code>requirements.txt</code> files, many of which share many dependencies between the various environment requirements files, so it would be nice if I can update my semantic versioning in one single location, instead of an variety of <code>requirements.txt</code> files.</p>