Logging: Add log levels to output

This commit adds log levels to the output. In systemd, it makes colored
lines, otherwise it's probably just a number. Bees is very chatty, so
this paves the road for log level filtering.

Signed-off-by: Kai Krakow <kai@kaishome.de>

test/chatter.cc

@@ -32,7 +32,7 @@ void
 test_chatter_three()
 {
 	cerr << endl;
-	Chatter c("tct");
+	Chatter c(0, "tct");
 	c << "More complicated";
 	c << "\ncase with\n";
 	c << "some \\ns";