Allow to customise the rendered output

As a user of redis-io, I'd like to be able to customise it's log output format to match the rest of the application (where we haven't adopted tinylog). Since redis-io relies on the tinylog API, the obvious place to allow for such customisation seems to be tinylog itself.

Note that I have opted for carrying the "renderer" (formatter) as a first class value also for the built-in formats. The drawback is that the netstrings settings getter doesn't make sense anymore, which however is a breaking API change.

For efficiency reasons, it may also be desirable to defer encoding of the timestamp and level fields to the renderer, as custom renderers likely want to rewrite / reorder those. As this would be a more intrusive change overall, I'm leaving it to a subsequent MR.