add optional attribute to Include type

see http://logback.qos.ch/manual/configuration.html

If it cannot find the file to be included, logback will complain by printing a status message. In case the included file is optional, you can suppress the error message by setting optional attribute to true in the <include> element.

<include optional="true" ..../>
Merge pull request #1 from rtack/patch-1

add optional attribute to Include type