Support customising the build for packages

[Nemo157]

For async-compression the majority of the API provided by the crate is disabled by default to reduce build time for dependencies. Given that, at the moment when this is built in Crater it is only really capable of checking the syntax of lib.rs, everything else is #[cfg]ed out.

It seems like there could be a package.metadata.crater section added similar to what docs.rs uses to configure necessary feature flags:

[package.metadata.crater]
features = [ "feature1", "feature2" ]
all-features = true
no-default-features = true
rustc-args = [ "--example-rustc-arg" ]
rustdoc-args = [ "--example-rustdoc-arg" ]

(I'm not sure if rustdoc-args could affect the doc-test build in some way, rustc-args could be useful in cases where a crate uses a non-feature cfg flag to enable some form of test support).

(related to #235)