Handle line markers

[MatthewHambley]

Fortran preprocessors will, by default, often include "line markers" in the processed source. These are used by compilers to give the correct line numbers in the unprocessed source when reporting issues.

They have this form:

# <line number> <filename>

fParser needn't do anything with them (although it might use them in the same way compilers do) but it would help if it at least recognised them. At the moment it means source which has been through the preprocessor can't be parsed unless the preprocessor has been told to omit markers.