Adding support for passing in an existing oauth2.Token in the App config

[thomas-tharp]

Support for using an existing OAuth2 token instead of authenticating via username/password.

Addresses #48

[turnage]

Not sure why CLA-Assistant isn't jumping on this one. If you wouldn't mind signing, this looks good to merge as is. Thank you for upstreaming your changes!

https://cla-assistant.io/turnage/graw

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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[Helyk]

@Odin243 Hi, I am interested in this PR and I will probably use it but there are a few things unclear to me.

1. How do you pass an existing token to the app?
2. Can the app perform the initial procedure of requesting authorization from a user? If not, how do you do it?