Add support for WIF strings

[ty-everett]

This is a simple change that enabled people to provide a WIF-formatted private key string as the second parameter.

I was having an issue where key instanceof PrivateKey was failing due to a complex peer dependency hoisting problem in my project (a [email protected] PrivateKey is not an instance of [email protected] PrivateKey).

This provides a fix, and also makes it more convenient for anyone else.