Add support for nasm?

[nirvdrum]

I was looking to use Platform IO IDE for debugging an assembly program, but it appears the assembler is hard-coded to as. I already have a project using nasm and can link the resulting object files into gcc. Rewriting the files to use syntax that as can understand is not really practical.

I modified the builder config locally and was able to swap in nasm without difficulty. It'd be helpful to do so without having to modify the PlatformIO source code.