Allow log levels to be configured per-source.

[jsthomas]

This change updates the logging API to allow log levels for sub-logs to be customized. This is a revision of #159, rebased on the latest in master for easier review. The change is motivated by issue #90; configurable log levels would assist with debug and warning logs for flash messages.

The change adds an optional argument to set the level of a sub-log at creation:

Dream.sub_log : ?level:[< log_level ] -> string -> sub_log

as well as a new function to set the log level afterward:

Dream.set_log_level : string -> [< log_level ] -> unit

Example Usage

let () =
  Dream.initialize_log ~level:`Warning ();
  Dream.set_log_level "dream.flash" `Info;
  let app_log = Dream.sub_log ~level:`Info "my_app" in 
  Dream.run
  ...
  (* Now log messages on my_app and dream.flash will show if they are of info-level or higher.
     Other messages must be warning-level or higher to be displayed.*)

[aantron]

Thank you!