Add support for G-code language

This is a special language controlling 3D printers (by RepRap, Makerbot,
Ultimaker etc.). It is not a general purpose programming language, but
still contains commands for e.g. looping. On the other hand, most of the
time it will be generated by another program, not hand-written. Hence I
classified it as "data".

Specification:
* http://reprap.org/wiki/G-code

Some repositories with examples:
* https://github.com/reprappro/Mendel
* https://github.com/BLLIP/bllip-parser
* https://github.com/MakerGear/M2
Update samples.json
Merge branch 'master' into 1530-local

Conflicts:
	lib/linguist/samples.json
Merge pull request #1534 from github/1530-local

1530 local
Add a multitude of Forth file extensions.
Yes, these are all in use.
Add Groff sample.
Update samples.json.
Make test_pedantic accept quotes around extensions.