[bug] `cargo-geiger` fails/errors out when `Cargo.toml` has nightly features

[Evrey]

To be precise, my Cargo.toml has cargo-features = ["strip"] to strip the symbol table in release builds. The TOML parser in cargo-geiger rejects this, assuming all Cargo.tomls to be stable only.

Here are my suggestions on how to solve this problem:

• Keep up with nightly and make the parser understand and accept nightly cargo features.
• Don't keep up with nightly and instead make the parser ignore unknown settings, assuming they are valid nightly things.

[anderejd]

For this specific problem, it's possible that this will be solved by upgrading our old cargo crate dependency (in progress).

I know next to nothing about nightly Rust since I don't use it due to its experimental and unstable nature, but if someone wants to add support for and maintain support for nightly that would be most welcome. To be clear, I don't have any philosophical problem with nightly Rust, it's a matter of resources (time).

[anderejd]

Upgrading cargo is in the works as part of #183.