Cache generated tokens between program executions.

[matecsaj]

Cache generated tokens between program executions to avoid the overhead of getting new tokens when old tokens have not expired.

User tokens last for months, so start with them.

As a security precaution, don't store the credentials used to get tokens in the cache file; perhaps supply the credentials to a one-way hash to create reproducible indexes.

Where should the cache be stored, and should the file be hidden?

If the scopes for the application token change, it will be necessary to flush the old token from the cache. Perhaps there are other reasons for doing a partial or complete flush of the cache. How will the flush be done?

Suggestions or a PR are welcome.

[matecsaj]

I've come to realize that not all users will want this. For those that do, I made some helpful changes.

0bf880c