Add CLI command to generate sample config

[arcz]

This changes the CLI interface of Echidna by adding echidna init command that generates a sample echidna.yaml config and moves the current fuzzing interface to echidna fuzz ... command:

Echidna

Usage: echidna [--version] COMMAND

  EVM property-based testing framework

Available options:
  -h,--help                Show this help text
  --version                Show version

Available commands:
  init                     Write a sample config file to echidna.yaml
  fuzz                     Run fuzzing

I reused the existing tests/solidity/basic/default.yaml, which already contains all the config options up to date.
I polished the contents by separating config options into logical groups and improved the descriptions. This file still requires work to be more user-friendly.

Additional ideas to consider:

• Load the echidna.yaml config by default
• Add path to echidna init <path>

Closes #940

[elopez]

Suggested change

	# Test mode, one of: property, assertion, overflow, exploration, optimization.
	# Test mode, one of: property, assertion, dapptest, overflow, exploration, optimization.