Inject accounts into `web3.eth.accounts` and web3.eth.getAccounts()`.

### What was wrong?

It's useful to use `web3.eth.accounts` for things like:

- list of accounts to select from.
- pre-check for some pre-condition that is account based.
- auto selection for some automated criteria.

Acocunts used with the new in-flight transaction signing middleware will not show up in this list.

### How can it be fixed?

Add a conditional to the constructor for the transaction signing middleware that when set truthy will inject any addresses it can sign for into the returned accounts list. Logic should include checks to ensure that addresses are not duplicated in the returned list.

link #649