Feature: Add ability to use Hardware wallets

[flofrie]

This is a feature request to add the ability to use hardware wallets with aut for signing Tx.

[fjribi]

@flofrie, the question is whether it is possible to make add an option to sign tx methods to support large variety of hardware wallets using ledgerblue? For instance, instead using a local key file, we select an option -hw in CLI commands with additional arguments (e.g: --path "m/44'/0'/0'/0/0" etc.) to wait for the hardware output (signed tx):

aut make tx | .atu sign tx -hw --path "m/44'/0'/0'/0/0" | aut send tx

[aiman]

This issue can probably be resolved by #197. Additional hardware wallets can be added on a case-by-case basis.