Implement method to list releases

[ibice]

Hi there! I added Updater.ListReleases to list available releases. I found it useful in case you want to update the binary to a specific version.

And, to reduce code duplication:

• Add repoInfo, a struct holding repository information and a constructor parseRepo.
• Add newRelease, a constructor for the Release struct.
• Add a function that returns the valid file suffixes, assetSuffixes.