The `--features` flag cannot be used multiple times

[madsmtm]

Running the following fails, while it would succeed were I using a normal cargo run.

cargo +nightly fuzz run my_fuzz_target --features=foo --features=bar

I know I can just combine this into --features=foo,bar, but for composability it would be nice if cargo-fuzz didn't require me to do so.

[fitzgen]

Agreed that we should match cargo's behavior here. Would be receptive to a PR that implemented this.