Add external .opcode.bin provision

[duncanamps]

Current status is that the assembler covers 8080/8085/Z80/Z180, these processors are baked in. Users wanting to experiment with other processors, undocumented instructions, etc., may not want or have the capability to recompile all the software.

Need to add an external .opcode.bin provision so if the user executes something like the following:

xa80 myfile.asm --com --processor=HD64180

As the processor is not baked in, XA80 should search for an external file either in the directory where xa80 was executed from, or failing that the current working directory. The file searched for should be HD64180.opcode.bin

It should also be possible to add a path, in which case it will just check this path and nowhere else, something like this should work:

xa80 myfile.asm --com --processor=/home/duncan/xa80/extras/HD64180

Also, this should be covered off by the XA80 environment variable if required.