Add warning for unknown sections in the manifest file

[adlerjohn]

Currently, unknown sections in the manifest file are silently ignored. Add a warning similar to Cargo's when an unknown section is found in the manifest file.

[eureka-cpu]

@adlerjohn if you have any resources on this I'd appreciate it, having trouble finding references to this issue online

[adlerjohn]

If you add a section

[[foo]]

to your Cargo.toml, cargo` spits out

$ cargo build
warning: unused manifest key: foo