Optional op_return output for generate_and_send_coins

[jachiang]

Allows us to conveniently generate op_return outputs with generate_and_send_coins: This would be useful for taptweak chapter mini case-study: #132

• Adds an optional data keyword to generate_and_send_coins:
• If data is set, an additional op_return(data) output will be created in the transaction at idx 1
• Builds on #142 (required) and #131 (optional).