Add a multitude of Forth file extensions.

Forth has a long a diverse history, and this is reflected in the file extensions in use.

In the wild:
ftp://ftp.forth.org/pub/Forth/Applications/

At GitHub:
http://github.com/wexi/amforth-shadow/tree/master/examples
http://github.com/AlexandreAbreu/jonesforth
http://github.com/liruoko/forth-exercises
http://github.com/earl/gforth-mirror