Support matrix result override

[wilsonehusin]

While we currently support individual overlay of OS and Arch, such change applies across all matrix result. I think there might be value in allowing overrides for matrix result too. Consider this scenario:

myprogram-Darwin-amd64.zip
myprogram-Linux-amd64-static.zip

I'm thinking it might look like this:

overlay:
  OS:
    linux: Linux
    darwin: Darwin
overrides:
  - OS: linux
    Arch: amd64
    Value: 'myprogram-Linux-amd64-static.zip'

Not ideal that we have to do this, but I can understand scenarios of CGO compilation and why maintainers choose this route.