logging: enable setting custom logging output func

To enable custom formatting of the log output while still using existing
backend, this commit adds the `log_output_custom` feature.
To use, register a commit with `log_custom_output_msg_set` then set the
log backend format set using `log_backend_format_set` with
`LOG_OUTPUT_CUSTOM`

Signed-off-by: Lucas Denefle <lucas.denefle@converge.io>