Fallback to standard string handling for unknown "format" values

[acgray]

Is your feature request related to a problem? Please describe.
If a string field uses an arbirtrary custom "format" value, as allowed by the OpenAPI spec, it is rejected by the parser.

Describe the solution you'd like
As it is guaranteed to be a string, even if the exact format is unknown, I suggest that the parser should follow the recommended behaviour in the spec and fall back to handling the field as a plain string. This is the behaviour recommended by the specification:

Tools that do not support a specific format may default back to the type alone, as if the format is not specified.

[dbanty]

Changes are merged in and will be available in 0.4.2.