Update FORTRAN file extension set

The standard extension for FORTRAN 2008 is .f08, so this is added.  In addition,  FORTRAN compilers support a number of special extensions that automatically trigger the preprocessor when compiling the FORTRAN code.  See http://gcc.gnu.org/onlinedocs/gfortran/Preprocessing-and-conditional-compilation.html for an example.

This commit simply adds these extensions so that when I open F90 files (in contrast to f90), it recognizes them as FORTRAN code properly.  I know I am likely an isolated use-case, and I've been using a modified geany for myself for a while, but I figured it was time I got to contributing...at least a tiny bit! 

Thanks!

(Also, sorry for not switching to another branch, but for this small change it shouldn't matter)