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 general, a search like this will find Forth code:
http://github.com/search?q=extension%3A.seq+dup+swap&type=Code

Repositories using these Forth extensions:
http://github.com/wexi/amforth-shadow/tree/master/examples
http://github.com/hanshuebner/rekonstrukt/tree/master/forth
http://github.com/gogonkt/LuneIME/tree/master/3rdparts
http://github.com/gnooth/forth/tree/master/tests
http://github.com/AlexandreAbreu/jonesforth
http://github.com/liruoko/forth-exercises
http://github.com/ekoeppen/CoreForth
http://github.com/earl/gforth-mirror
http://github.com/eungju/hf86
http://github.com/keithcausey/GreenArrays/tree/master/EVB001/pF
http://github.com/yashpant/AutoPlug2.0/tree/master/AutoNet_Current/EVM/Forth%20Interpreter%20%28Reference%29/Forth/Forth/WINFORTH
http://github.com/kt97679/relf
http://github.com/cataska/ficl/tree/master/softcore