Disambiguate .fs between F#, Forth, and GLSL

The `.fs` file extension is commonly used for both F#, Forth, and GLSL. Currently, only F# is detected.
(A previous incarnation of this pull request was #646.)

GitHub currently contains about 48000 files with an .fs file extension, which are detected as F# or OCaml. Of those, I believe 15000 files are actually in Forth, and at least 13000 are GLSL (but probably more).

This pull request adds sample files, heuristics, and .fs as Forth and GLSL extensions.

Tested with a corpus of 1000 `.fs` files provided by @arfon.

Examples of misidentified code at GitHub can be found with these searches:
http://github.com/search?q=extension%3A.fs+dup
http://github.com/search?q=extension%3A.fs+void+main