Allow more Management Plugin log configurations

[inikulshin]

Currently, only http_log_dir is supported.
However, most (all?) lager file backend settings (name, date, size, count, formatter, ...) are relevant in this case. Especially count to avoid creating thousands of log files (that's the reason I'm starting this discussion).

I suppose, the best solution will be to use lager in webmachine_log.erl instead of file:write.

[michaelklishin]

Not lager but the standard Erlang 25+ logger, please. It supports formatting configuration to some extent. Not everything in the standard logger works exactly like in Lager but in general, they are close to feature parity from what I know.