Added basic CLI functionality based on click module.

[malekinho8]

Added a method to Docs object that allows user to enter 'chat' mode, which is basically just query on an infinite loop. Reflected changes in setup.py to allow for easy installation. Updated README.md to include a Usage example.

I tried to implement changes based on your suggestion, but not 100% sure if I did everything right. For now I put cli.py in the paperqa.contrib folder which houses the code to enter into chat mode with the Docs object from the CLI. LMK your thoughts/next steps whenever you get a chance!

[MilesCranmer]

Good idea. Might be nice to allow an arbitrary number of arguments? Then one could combine with shell globbing, like:

paperqa my_pdfs/**/*.pdf

[malekinho8]

@MilesCranmer Just finished adding your suggestion!

[jamesbraza]

Hello @malekinho8 thank you for this PR. Version 5 (just released) ships with a CLI, so I am going to close this out.

We appreciate your help and please feel free to continue opening PRs oriented towards the new CLI