TeeLog: Allow arbitrarily many base logs

[TheBB]

Should combine nicely with #29 to let you write:

TeeLog(
    FilterLog(DataLog(), minlevel=proto.Level.info),
    FilterLog(StdoutLog(), minlevel=proto.Level.info), maxlevel=proto.Level.user),
    FilterLog(StderrLog(), minlevel=proto.Level.warning),
)

[gertjanvanzwieten]

Sorry for the late reply, this fell off the radar. In light of other work we're a bit hesitant at the moment to make changes that we might have difficulty backtracking from if needed, so we prefer to not merge this right away. I'll keep it open though for when that situation clears up.