Implement missing gcodes that affect position

[remcoder]

done in v3.0:

• [x] G20 - Set units to inches
• [x] G21 - Set units to mm
• [x] G28 [X Y Z] - Home axis

planned for v3.1+

• [ ] G90 - Absolute positioning mode
• [ ] G91 - Relative positioning mode
• [ ] G92 [X Y Z E] - Set current position
• [ ] M206 [P T X Y Z] - Set home offsets
• [ ] M428 - Home Offsets Here
• [ ] absolute E distances

[sophiedeziel]

My parser refactor is not so unnecessary it seems! I'll try to split that in a new branch.

I have some state management that will be useful.

[remcoder]

There's a new architecture with a revamped parser in v3.0 so let't move this to v3.1+, apart from the commands that are already in v3.0.