Add NASM (Netwide Assembler) language definition.

I'm not sure how to work new definitions into the build system, so I didn't make those changes.

This is quite far from complete, and I think looks a bit strange with the default color scheme, but it works.  A better solution would be to actually code in all the instructions so you could catch things like labels without ending colons, etc.  However that's a lot more work - this is the 80% solution.

I decided to choose NASM instead of asm-intel due to the number of different dialects of assembly language (which is important with more advanced features - but I haven't implemented those).  I'm not dead set on that though.