Add helper that closes a user's account

[morleyzhi]

https://stellar.expert/demolisher/public/

• Automatically closes open offers.
• Automatically sells owned assets on Stellar DEX at market price.
• Automatically removes trustlines, returning all unsold assets to the issuers.
• Automatically removes existing data entries.
• Allows merging directly to exchanges and other destinations that do not support merge operations out of the box.
• Works with multisig accounts.