Customize logger ?

Hello,

**Is your feature request related to a problem? Please describe.**
Is it possible to customize logger ? I feel like `drogon` is using `trantor`'s logger and we have to use it. I would like to know if it's possible to override this logger by another, let say a `spdlog` logger for example, that would be user-defined.

**Describe the solution you'd like**
Be able to pass a custom logger to `drogon`, that it'll use for internal log.

**Describe alternatives you've considered**
Only use `drogon`'s logger or use 2 loggers in the project.

Maybe it's possible by redefining the macros `LOG_INFO`, `LOG_DEBUG`..? But I don't know if it's possible / desirable.

Thank you, regards