Provide better release filenames

[max-wittig]

First of all. Thank you for this tool!

Just noticed that usingmanifest-tool e.g. in a CI pipeline, one needs to provide the version several times.

script:
- apt-get update -qq && apt-get install -qq curl
- curl -s -L https://github.com/estesp/manifest-tool/releases/download/v2.0.0/binaries-manifest-tool-2.0.0.tar.gz -o binaries-manifest-tool.tar.gz
- tar xzf binaries-manifest-tool.tar.gz
- ./manifest-tool-linux-amd64 --username ...

Also it would be nice, if binaries could be provided directly to download

[thesayyn]

Also it would be nice, if binaries could be provided directly to download

I think this would be better as well. Perhaps their signatures could be provided to allow users to do integrity checking.

@estesp I'd be more than happy to work on this.

[pjastrzabek]

Right now all binaries are packed together in a tar.gz file.
If i want to use it in docker I have to download all, unpack, take 1 and delete the rest.
Having an option of downloading only one binary for architecture that I need, would be tiny improvement.