Commands:

WITHERLOG logtype [type]
witherlog.logtype
Toggles logging for the specified type. If you omit type, it reports what types are active.

WITHERLOG logfile filename
witherlog.logfile
Sets the file to log to, if file logging is enabled

WITHERLOG reload
witherlog.reload
Reloads config from disk

Miscellaneous:

There are two vars you can use in messaging; %date% and %time%. If you want to localize the format, you can set it in the config. The default is a more world centric style, but like if you wanted a typically American format you could do this for example:
You can find codes to use here and if that page dies, just Google "Java SimpleDateFormat codes" and you'll likely find another pretty easily.

It determines the players to report based on whoever is within a range of where the wither spawned. Default is 6 x 6 x 6 blocks. If you want to adjust it to like 10 x 5 x 10, you would do:
With the 1.1 update, I made it more specific about player matching, keeping tabs on who is placing Wither skulls on soul sand/soil. The way it matches them up is it loops every player within range of the new Wither and if they appear in that list of people who placed skulls, it reports them and them alone. If for some reason nobody near is in the list, it falls back on reporting all nearby. The time latitude by default is 1.5 seconds from placement to Wither spawn. Should be fine, but if you need to find tune it the path is:
That would allow for 10 seconds between skull placement and Wither birth to associate players. Shorter would be more accurate, longer less so.