[Feature request] - Limiter global exempt_when option

[ecly]

It would be useful to be able to provide a global/default exempt_when option, for when using a limiter with application_limits or default_limits.

Especially together with: #160

Would allow to e.g. apply a global limit for unauthenticated users and then make an exemption when authenticated.

If this is a welcome feature, I'd be happy to make a PR.