Add support for other account types.

[benoitdevos]

• Add support for other account types
• Add method fromUnknown() - aiming at detecting account type from address.
• Provide strong validation of Bech32 addresses - otherwise almost any dummy addresses were accepted as valid Bech32 addresses. (using bitcoinjs/bech32)
• Provide a stronger validation for Polkadot addresses - for some reasons the previous implementation using validateAddress() used to validate addresses that were later on unable to decode. The new implementation is strongly inspired from checkAddress()

logion-network/logion-internal#1216

[gdethier]

Good job!