Make "homepage" optional in Cargo.toml

[bunburya]

This tool appears to fail if the homepage field is not specified in Cargo.toml. C-METADATA, which is linked in the README, states that homepage is optional and further states:

The homepage metadata should only be set if there is a unique website for the crate other than the source repository or API documentation. Do not make homepage redundant with either the documentation or repository values. For example, serde sets homepage to https://serde.rs, a dedicated website.

Given that a lot of crates these days will just have a repository URL and not a separate homepage, I think it would be good to make homepage optional in cargo-aur (following the approach in C-METADATA). The url field in the PKGBUILD could be set to homepage or, if that is not present, documentation or, if that is not present, repository (the last being a required field per C-METADATA).

Happy to work on a PR for this if helpful.

[fosskers]

Great suggestion. Want to give it a shot?