Introduce undocumented option logger

This commit reworks the logging that Cowboy does via
error_logger to make the module that will do the actual
logging configurable.

The logger module interface must be the same as logger
and lager: a separate function per log level with the
same log levels they support.

The default behavior remains to call error_logger,
although some messages were downgraded to warnings
instead of errors. Since error_logger only supports
three different log levels, some messages may get
downgraded/upgraded depending on what the original
log level was to make them compatible with error_logger.

The {log, Level, Format, Args} command was also
added to stream handlers. Stream handlers should
use this command to log messages because it allows
writing a stream handler to intercept some of those
messages and extract information or block them as
necessary.

The logger option only applies to Cowboy itself,
not to the messages Ranch logs, so more work remains
to be done in that area.

doc/src/manual/cowboy_stream.asciidoc

@@ -81,9 +81,8 @@ Stream handlers can return a list of commands to be executed
 from the `init/3`, `data/4` and `info/3` callbacks. In addition,
 the `early_error/5` callback must return a response command.
 
-// @todo We need a 'log' command that would call error_logger.
-// It's better than doing in the handlers directly because
-// then we can have other stream handlers manipulate those logs.
+// @todo The logger option and the {log, Level, Format, Args}
+// options need to be documented and tested.
 
 The following commands are defined: