Improve logging output handling

[Alaettin Serhan Mete]

Currently we're creating an spdlog::sinks::basic_file_sink_st with a hardcoded name, prmon.log. As a result, the file gets overwritten when prmon is executed multiple times in the same location. This happens, for example, in the ATLAS production environment when a job has multiple execution steps. It would be ideal to a) make the log file name configurable and b) avoid overwriting existing logs.