Add support for providing a private key instead of mnemonic to run an element node

[gjgd]

I'm following instructions here and at some point the docs ask to provide a mnemonic. It's a bit inconvenient because:

• Don't want to provide my metamask mnemonic which has mainnet ETH on another address
• It's inconvenient to create a separate mnemonic because metamask only supports having one mnemonic, so would have to manage it some other way.

Instead, would it be possible to use Element using an exported private key? Would be easier because then people could create a separate address on metamask for Element, and export the key for running the node, while still maintaining separation with other addresses.

[gjgd]

Will send a PR sometime this week