#9 Section <include> improvements

[artegz]

1. Currently it is possible to use system properties in property values. It would be great to be able to use system properties also in 'name' attribute of <include> tag (and probably in some other places).</include>

2. It would be nice to have 'optional' attribute for <include> tag. If it's set to 'true' then included config may not exists. </include>

3. Currently parser process 'include' properties before 'sections' properties, so 'sections' properties override same properties from 'include'. It will be useful to make overriding behavior configurable - to be able to override 'sections' properties with 'include' properties' somehow.

I would suggest a patch where first two points have been implemented (see attachment).