Allow setting options to cargo fuzz run -- [HERE] closer to the source

[Ekleog]

Hello,

Right now, I have a fuzz/fuzz_*.sh directory, that calls into the right fuzz targets the right way (full source at https://github.com/Ekleog/yuubind).

With #42 fixed, I could remove half those scripts (ie. the half that cds into the correct folder).

However, it would still leave setting the correct -dict="../fuzz/$DICT option in the scripts.

I'm thinking that it'd be great if it were possible to set the libFuzzer arguments closer to the source code, eg. by having them in smtp-message/fuzz/Cargo.toml. With that change, the user would only have to set --jobs manually, which sounds pretty reasonable :)

What would you think about that?

Cheers, and thank you for this neat piece of software :)