Support Python packages that aren't on PyPI

[maxfisher-g]

Packages such as https://github.com/ytisf/PyExfil have not been updated on PyPI and the current installation instructions recommend cloning from GitHub and running pip setup.py install. It would be nice if we could support analysis of Python packages with this kind of installation flow.

It's also released as a .zip file with a nonstandard structure

[dukecat0]

pip setup.py install is not a valid command.
Usually pip install git+https://github.com/ytisf/PyExfil.git should work if the package needs to be installed from GitHub. This command will install it directly without needing to clone and run install commands manually.

[maxfisher-g]

Good point @dukecat0. I guess in general it would be nice to be able to override the install command in certain cases. This will be enabled by #301 (i.e. copying the analyze script into the sandbox at runtime rather than embedding it into the sandbox image)