Add Configuration File Validation and Error Handling

BeEF should validate the configuration file options and exit upon failure.

Each extension should be able to define it's own validation for configuration file options, which is called upon extension soft load.

This is especially important to prevent issues arising due to changes to the configuration file which affect backwards compatibility.

Trace the existing configuration loading path and the extension soft-load entry point, then inspect how configuration options are currently defined. Done means extensions can validate their own options and the application exits with an error when validation fails, including cases affected by backwards-incompatible configuration changes.

Written by the indexing model from the issue text.