Create Global Config and Generic Backend Loading from toml

[BradenEverson]

• ✨ Create Config struct that currently just holds a Vec<BackendConfig> but could be upgraded to hold more config options later.
  • By default, Config can be loaded globally from ~/.crankshaft, but there also exists the method to manually specify a config path
• ✨ Create Generic and Docker backend configurations, Docker is currently unimplemented.
• ✅ Create tests for config loading and submitting to a generic backend
• Note: Currently BackendConfig holds a simplified submit method I created just for testing the parameter replacing and script running. Once the Backend trait is created this method will be removed, and the script running functionality will be added to GenericBackend's submit implementation