Add `rustfmt.toml`

[nuke-web3]

Motivation

Substrate added rustfmt.toml recently that I would propose is used here as well to keep consistant formatting throughout parity code bases, at least those built on (or for) substrate.

Suggested Solution

Follow the same CI flow and use the same formatting file https://github.com/paritytech/substrate/blob/master/rustfmt.toml

Alternatives

Use substrate's formatting as a base, and customize as the maintainers here see fit.

[sorpaas]

I'll handle this myself. Saves reviewing time.

[nuke-web3]

Fixed in #432 🎉