Support 'INC D' and 'DEC D'

[JayFoxRox]

Implemented as found in DSP56000 manuals.

Tested using JayFoxRox/xbox-tools#70 with following test setup:

# Generate some test cases
sr = 0xC00310
inputs = (
  {'a2':0x00, 'a1': 0x001000, 'a0': 0xFFFFFF, 'b1': 0x10, 'sr': sr },
  {'a2':0x80, 'a1': 0x002000, 'a0': 0x000001, 'b1': 0x20, 'sr': sr },
  {'a2':0xFF, 'a1': 0x003000, 'a0': 0xFFFFFF, 'b1': 0x30, 'sr': sr }
)
processing = ['inc a'] * len(inputs)
outputs = [['a2','a1','a0','b1','sr']] * len(inputs)
tests = list(zip(inputs, processing, outputs))

More tests were done using 'inc b', 'dec a', 'dec b'.

Closes #8