add configurable username claim

[jeff-sternberg]

This is a suggestion to fix #213 that is similar to #224, but provides a configurable username_claim attribute.

Logic:

1. Read value of c.AzureAdOAuthenticator.username_claim
2. Fall back to oidname if config isn't set

closes #224

[betatim]

Thanks for the quick turn around! This PR looks ready to go after my double check comment. I'd merge this as is and then we can have a second PR to switch over get_tenant (or make an issue for later).

[minrk]

Fall back to oid if config isn't set

Looks like the fallback is to name if it isn't set. Does this PR mean to change this to oid?

[jeff-sternberg]

@minrk originally yes that was the intent, but as @betatim suggested, if we fall back to name that will be backwards compatible. Also with this change, a user can configure oid if desired.

[minrk]

Sorry, should have read through better. Thanks!

[jeff-sternberg]

No worries, and thanks for merging!