Set nonce as "next including unconfirmed transactions"

[iamdefinitelyahuman]

Overview

It should be possible to broadcast a tx and set the nonce as "the next available nonce, including all the pending transactions".

Specification

Nonce is already exposed since #488, but the value is currently given as an integer.

I have a few ideas but not sure which is the best. Open to comments or further suggestions.

accounts[0].transfer(accounts[1], nonce="next")
accounts[0].transfer(accounts[1], nonce="pending")
accounts[0].transfer(accoutns[1], nonce=accounts[0].pending_nonce)

Dependencies

Requires #587 before it will be useful.