src/main.sh: Add --experimental-features and --extra-experimental-features support

[ShamrockLee]

• Default to using --extra-experimental-features instead of --experimental-features to show respect to the environment configuration.

• Add flags --extra-experimental-features and --experimental-features to allow manual specification to the experimental-features setting (e.g. use builtins.getFlake when not globally enabled).

Fixes #35

[Kranzes]

@msteen Can you please merge this and tag a new release?

[ShamrockLee]

Describe the changes inside the CHANGELOG.md for version 0.4.2.

[ShamrockLee]

@msteen Could you help take a look?

[ShamrockLee]

I accidentally base the CHANGELOG.md work on a non-relevant commit. Just solve the problem.

[jakubgs]

Any news on this? I'd love to be able to generate a new vendor SHA for packages built with buildGoModule in a flake.nix.