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

lib/linguist/languages.yml

@@ -756,6 +756,14 @@ Frege:
   extensions:
   - .fr
 
+G-code:
+  type: data
+  lexer: Text only
+  extensions:
+  - .g
+  - .gco
+  - .gcode
+
 Game Maker Language:
   type: programming
   color: "#8ad353"

samples/G-code/duettest.g

@@ -0,0 +1,57 @@
+; RepRapPro Ormerod
+; Board test GCodes
+M111 S1; Debug on
+G21 ; mm
+G90 ; Absolute positioning
+M83 ; Extrusion relative
+M906 X800 Y800 Z800 E800 ; Motor currents (mA)
+T0 ; Extruder 0
+G1 X50 F500
+G1 X0
+G4 P500
+G1 Y50 F500
+G1 Y0
+G4 P500
+G1 Z20 F200
+G1 Z0
+G4 P500
+G1 E20 F200
+G1 E-20
+G4 P500
+M106 S255
+G4 P500
+M106 S0
+G4 P500
+M105
+G10 P0 S100
+T0
+M140 S100
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+G4 P5000
+M105
+M0
+
+
+
+

samples/G-code/square.g

@@ -0,0 +1,13 @@
+G28 X0 Y0
+G1 X55 Y5 F2000
+G1 Y180
+G1 X180
+G1 Y5
+G1 X55
+G1 Y180
+G1 X180
+G1 Y5
+G1 X55
+M0
+
+