Add F# and GLSL samples.  Add Forth and GLSL extension .fs.
Add heuristic to disambiguate between F#, Forth, and GLSL.
Merge pull request #1758 from larsbrinkhoff/fsharp-glsl

Disambiguate .fs between F#, Forth, and GLSL
Add Forth extension .fr; and a sample.
Add 'text' language and heuristics for .fr.
Add .html.fr and .xml.fr; plus samples for .txt.