Accept all possible private key encoding types (base64, hex, wif)

[pshenmic]

Issue

We had only WIF encoding format for a private key, but user could have one in HEX encoded, and even Base64. It should be possible use any of them in the private key file transparently for the user.

Things done

• Introduced new required command flag network
• Added a decoding function that tries to validate and load private key from any of known encoding types
• Corrected few flags documentation