Allow local binary to be renamed.

[Integralist]

Problem: as we integrate a new binary for C@E local testing we don't want to confuse users by downloading the binary using its internal name.
Solution: support renaming the binary once it has been extracted from the release. Something like...

org := "fastly"
repo := "viceroy"
binary := "viceroy"

viceroyVersioner = update.NewGitHub(context.Background(), org, repo, binary)
viceroyVersioner.RenameLocalBinary("fastly-localtesting")

[kailan]

My thoughts on this as discussed with @Integralist:

• If Viceroy is being pulled from GitHub releases, as this PR suggests, then it seems like the binary being named "viceroy" would not be an issue. A quick search for "fastly viceroy" will answer the question of what it is.
• If Viceroy is staying as a private repo, I would suggest renaming the binary on the update server rather than downloading it and then renaming locally.

Out of interest, where will viceroy be downloaded to? I assumed the config dir (~/Library/Application Support/fastly on macOS), in which case I would not worry about file naming. If they are confident enough to poke around in system dirs and delete a random binary, it should be obvious what has happened when fastly compute serve stops working.

[cratelyn]

This looks good to me! While I think I do agree with @kailan's point above, the ability to rename a local binary ourselves seems like a worthwhile addition.

[Integralist]

Thanks @cratelyn

@kailan I think under the circumstances I'm OK to merge this PR then. Just wanted to give you one last opportunity to holla if you were strongly opposed.