This PR adds three different strategies for generating fuzzing harnesses via the template command. Related to #37

The three modes are:

• simple - The fuzzing harness will be generated with all of the state-changing functions from the target contracts. All function calls are performed directly, with the harness contract as the msg.sender.
• prank - Similar to simple mode, with the difference that function calls are made from different users by using hevm.prank(). The users can be defined in the configuration file as "actors": ["0xb4b3", "0xb0b", ...]
• actor - Actor contracts will be generated and all harness function calls will be proxied through these contracts. The Actor contracts can be considered as users of the target contracts and the functions included in these actors can be filtered by modifier, external calls, or by payable. This allows for granular control over user capabilities.

Changes

• Added a new CLI flag --mode to the template command. Added three modes: simple, prank, and actor
• Updated the HarnessGenerator class to enable mode selection
• Fixed harness test
• Updated README
• Updated template to import Hevm from properties