Add pre-buy and post-buy hooks

[tspoff]

• Allows for arbitrary logic contracts that are hooked into before and after the buy function

These allow dynamic expansions of bonding curve functionality.

• It calls the external hook contracts in a loop on the given event.
• There are no parameters passed in at the moment, but we could serialize and deserialize a bytes array.
• They are capable of reverting the contract or modifying the bonding curve contract variables.