Allow transaction signing

[AlanVerbner]

Description

web3-repl should allow users to be able to sign and broadcast transactions. Two different modes are interesting:

1. Password prompt mode: User will be requested for private key input in after sending the command using bip39
2. Allow users to preload their private key and automatically sign transactions

$ cat myPrivateKey.txt | web3-repl -p http://localhost:8545
> web3.eth.sendTransaction({...}) // transaction gets signed and broadcasted