Allow log levels to be configured per-source.

[jsthomas]

This change proposes an update to Dream.initialize_log that allows log levels to be specified for individual log sources.

This is motivated by comments in #90. There, @aantron mentioned the need for a way to configure log level per source, which would be useful for working with logs related to flash messages.

Example Usage

let () =
  Dream.initialize_log
    ~level:`Warning
    ~custom_levels:[("dream.flash", `Info)]   (* NEW *)
    ();
  Dream.run
  ...

The change introduces an optional argument called custom_levels that accepts an association list. Sources not named in ~custom_levels are set to ~level, so Dream.initialize_log retains its original behavior when ~custom_levels is not used.

[aantron]

I would suggest a separate API like

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

That would allow different modules to "locally" set log levels for sources that are relevant to them, and not have these concerns centralized in the log initializer function (which might not even be called for any other purpose).

It might also be good to add ?level to Dream.sub_log, so:

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

[jsthomas]

In the interest of simplicity, I'm going to close this PR and open a new one that has the latest changes on the master branch.