Manifest file -based strategy

[pjcdawkins]

Thanks for this library - I am going to use it instead of the abandoned project https://github.com/kherge-abandoned/php-phar-update. One advantage that had was to grab available versions and SHA1 hashes from a single "manifest" file.

I've written a custom strategy to handle this for our project:
https://github.com/pjcdawkins/platformsh-cli/blob/replace-phar-update/src/SelfUpdate/ManifestStrategy.php

and here's what the manifest file looks like:
https://platform.sh/cli/manifest.json

It seems like this could be generalised for anyone, would you be interested if I made a PR?

[pjcdawkins]

I can envisage more ideas here so I'll make a PR when I've got them ready