Allow header/footer in DDLogFormatter

[chrisdolan]

In some log file syntaxes (like XML) it's useful for the log formatter to support a header and footer method that are invoked at the beginning and end of the log stream. For plain text log formats, these will be the empty string.

Please either add header and footer methods to the DDLogFormatter protocol, or add a new protocol with those methods that is checked and invoked when a new log file is opened or closed.

Consider the Layout interface of the Java Logback framework: https://github.com/ceki/logback/blob/master/logback-core/src/main/java/ch/qos/logback/core/Layout.java

[robbiehanson]

The log file syntax (like XML) should be the responsibility of the logger, not the formatter.

Take the XML example: One could extend the DDFileLogger class (DDXMLFileLogger) and automatically append the header and footer.