add options to pass features in mutagen runner

[samuelpilz]

Fixes first part of #154.

This adds flags --features and --all-features to the runner in the same way cargo handles them.

[llogiq]

This could be an optional dependency.

[samuelpilz]

I tried that, but cargo responded with: "Dev-dependencies are not allowed to be optional: mutagen"

[elichai]

rust-lang/cargo#1596

But because it's an example I think you can move it to a regular dependency

[samuelpilz]

One thing to consider: Mutagen would then be included when running cargo build --release --all-features. However, in the current form such call would fail.

Please note that this is an example and people will figure out how they want to use mutagen themselves. If anyone really feels strongly about this issue, I can adapt the example.

[llogiq]

Nah, in that case it's OK.

Interesting. I think that's a missing cargo feature.

[elichai]

Nice. Thank you!