Add a way to config publish targets for different make targets

[MarshallOfSound]

Basically when #116 and #117 are sorted we will need a way to apply publish targets to different make targets through config.

Current Technique:

electron-forge publish --target=bintray

Now that isn't too bad but I think we can do better 😄

Proposal:

{
  "publish_targets": {
    "win32": ["github", "s3"],
    "darwin": ["github"],
    "linux": ["github", "bintray"]
  }
}

[malept]

Very similar to make_targets, 👍

Although I wonder what the use case is for having different publish targets for different platforms.

[MarshallOfSound]

@malept I for instance would want to publish linux artifacts to bintray and github while the rest only to github.

[malept]

Good enough for me 😄