Option to reduce log verbosity

[james-ctc]

It would be nice to have a compile option so that file and line numbers are omitted for the lower level logs.

[hikinggrass]

I'm working on moving over the logging backend from Boost.Log towards spdlog at the moment. I already added a way of overwriting file/line/function per individual log message (could be useful if the logging comes from another place). But I'll add a compile time flag for disabling this entirely. Another option might be to (additionally) provide individual formatstrings for the different log levels