JWT Oauth Flow?

It looks like ex_force doesn't presently support the JWT Oauth flow offered by Salesforce... I use this authentication flow quite often connecting to Salesforce for integration projects - it would be useful for me and perhaps others.

I am an experienced Salesforce developer, but a beginner with Elixir. I would be happy to try and get a PR going to add JWT support to ex_force, if you would be open to it - what do you think?

Thanks!
Scott