Allow choosing a custom HD derivation path

[BlinkyStitt]

Similar to LedgerHQ/ledger-wallet-ethereum-chrome#7

I was using Trezor to manage my funds and it uses m/44'/60'/0'/0, but this repo looks like it is using m/44'/60'/0' for the path. This repo also is hard codes the first address, but I have funds spread across multiple addresses. I think this repo picked this to be compatible with ledger's app.

Rather than just adding a /0 to the hard coded path which would break existing user's setups, I think that there should be a setting somewhere to choose from paths just like https://www.myetherwallet.com/ has.

For now I think I can just modify config/main.json myself and self-host.