Allow batch compiler to ignore a missing preferences file

When the Eclipse Java Language Server analyze a multi-module Maven project, opened in an IDE, the lack of a .settings/org.eclipse.jdt.core.prefs is silently ignored, as if this file existed but simply had no contents.

However, if we attempt to compile the same project tree with the batch compiler, the same set of command-line arguments cannot be used for all of the projects; any project missing a preferences file will cause the compile to halt with an error.

Pull request #176 adds a variant of the `-properties` command-line argument that _optionally_ takes a file, but does not complain if it does not exist.