Allow customization of log levels

[turing85]

Is your feature request related to RFC7807? Please describe.
No.

Describe the solution you'd like
Currently, the ProblemLogger logs everything to log level INFO when the response is < 500 and everything >=500 on level ERROR.

It would be nice to:

• have a way to turn off logging completely (without setting the min-level for http-problem to FATAL),
• be able to define a default log level, and
• to be able to define a log level per response code and/or response code range (e.g. 401 and 403 on level WARN, everything else < 500 on level INFO, everything >= 500 on level WARN)

Additional information
This is slightly related to #512, but #512 focuses on stack traces, while this issue focuses on the log level.

[turing85]

I have some ideas to implement this feature. I am willing to send a PR if you give me the go-ahead.

[melloware]

@turing85 i would say submit your PR