Generate default shard.yml when missing

[crystal-lang]

Generate default shard.yml when missing

[ysbaddaden]

Shards is now doing its best to generate a fake shard.yml when missing, by extracting the version from git tags, and GitHub dependencies from Projectfile.

It's a best effort and is likely to work in most cases but not in corner cases. For example, the Projectfile is parsed with regular expressions for strings and not compiled by Crystal, and it will only work for the given cases:

deps do
  github "user/project"
  github "user/project", branch: "name"
  github "user/project", "name"
end

[asterite]

@ysbaddaden This is awesome!! This means that if we decide to change crystal deps to use your shards project, the migration will be painless.