[photran] Hook for custom preprocessor

[Mike Garrahan <mgarraha@xxxxxxxxx>]

I'm working on a Fortran program which uses a homegrown preprocessor. It
has directives of the form ##IF etc., which cause Photran to complain of
unexpected extraneous characters. What are the most natural extension
point(s) to enable analysis and refactoring on such code? Will this
change much in version 5?