Fixes for #629 and #630 (custom line processor functionality)

Made the updates based on my discussions with @khellang.
ServiceOverrides now stores the ILineProcessor list in the overrides dictionary.
FilePreProcessor.IsNonDirectiveLine now considers custom IDirectiveLineProcessor implementations.

Solution builds, all tests are green, and the executable works with my custom module and script.  Let me know if any further changes are required.