Add a way to override content types

[dbanty]

Add content_type_overrides or similar to the formats accepted by --config so that folks can redirect a specialized content type to one of the implementations that exist in this project.

Specifically, one might solve #592 by adding something like:

content_type_overrides:
    "application/vnd.api+json": "application/json"

See #655 for more info.

[m32]

some identifiers (tags) may contain long strings with national diacritics:
Interfejsy interaktywne - poświadczenia

identifiers with multiple dots inside them can also be long:
online.credentials.credentials.context.grant
online.credentials.credentials.context.revoke

so it would be useful to have the ability to overwrite them:

tag_overrides:
Interfejsy interaktywne - poświadczenia: poswiadczenia

operationId_overrides:
online.payment.payment.identifier.get.reference.numbers: get
online.payment.payment.identifier.request: request

[dbanty]

Done in 0.19.1