Add a signer provider selector

[onbjerg]

Should be possible to choose what Web3 provider is injected in place of MetaMask:

• Private key (should also note that this is insecure)
• Ledger
• OS signer (when that exists)

[luisivan]

Why not just resorting to the OS signer, which then can resort to private key/Ledger/etc?

[onbjerg]

Because it doesn't work yet, so this is a temporary measure :p

[luisivan]

Oh, try the new version, it even works with Ledger:tm:

[onbjerg]

I know it works with Ledger, but it only runs Ganache. No way to switch to a remote node right now.

[luisivan]

Gotcha. Should be trivial to implement though

[onbjerg]

Ya

[luisivan]

Closing in favor of aragon/client#374