Better support for compound loggers

[pyricau]

Related to #22 and @ericmaxwell2003's feedback: while one can set up a custom logger that knows how to swap delegate loggers in and out, one challenge we found is that the API is split between "isLoggable" and "log". When delegating to multiple loggers, in the "log" call we have no idea which logger said yes to "isLoggable".

Not sure how to address this exactly, but the current API makes compound loggers awkward.