Implement LOADSUB Operation

[craigthomas]

The LOADSUB operation loads memory pointed to by the index register into a subset of registers specified by the programmer. The mnemonic is as follows:

LOADSUB r1,r4

Only registers are valid operands. It specifies a register range that the programmer wishes to load memory into. Valid registers are r0 through rf. The mnemonic is encoded as follows:

5XY3