[proposal] activate testnet mode via configuration

[krtschmr]

i've expected a configuration where we simply say

config.testnet = true

we then don't need to pass arguments into the address generation anymore

def to_serialized_hex(type = :public, network: :bitcoin)

could be changed to

 def to_serialized_hex(type = :public, network:  nil)
   network = config.network unless network

what would be against this idea?

would also benefit for testing purposes

[krtschmr]

why this won't be merged? it's super useful.

also i propose that testnet is running by default