FIX: .f misidentified as Fortran

More than 500 Forth files with the `.f` extension are misidentified as Fortran:
http://github.com/search?q=extension%3A.f+dup+swap

This pull request adds
- `.f` as a Forth extension
- Fortran samples
- Heuristics to chose between Forth and Fortran

Tested with these repos:
http://github.com/ruv/epurger
http://github.com/certik/fortran-utils
http://github.com/joamaki/jforth
http://github.com/nschloe/lapack
http://github.com/besser82/linpack
http://github.com/xianyi/OpenBLAS
http://github.com/robertord/SBP-MCAP
http://github.com/zwizwa/staapl
http://github.com/voidlizard/tdforth