Feature rename axis

Add the ability to rename the x, y, or z axis to any arbitrary string.

EX:

```python
>>> g.rename_axis(z='A')
>>> g.move(10, 10, 10)
G1 X10 Y10 A10
```

@danfitz7 This is the first step in implementing nozzle changes, the rest will be built on top of this and is coming soon.