Please introduce reset command to clear credential cache

[wora]

For security purpose, oauth2l should have a reset command to clear credential cache.

[bochunz]

I will keep this until we add revoke the cached token. Now it only delete the cache file.

[dclendenan]

It would be very useful to add an 'ensure' function that would reset/refresh the token if expiry was below some threshold (with a sane default time). This looks like a reasonable extension, I'd be willing to submit a PR if this would be accepted as a workflow.