Make "homepage" optional in Cargo.toml

This tool appears to fail if the `homepage` field is not specified in `Cargo.toml`. [C-METADATA](https://rust-lang.github.io/api-guidelines/documentation.html#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.