[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: https://github.com/laurentS/slowapi/pull/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.

[laurentS]

Hi @ecly although this issue is more than a year old, I think the change you propose would be valuable. Are you still keen on making a PR? (please note #232)