The 'rpc_command' hook allows a plugin to take over any RPC command.
It sends the complete JSONRPC request to the plugin, which can then respond
with :
- {'continue'}: executes the command normally
- {'replace': {a_jsonrpc_request}}: replaces the request made
- {'return': {'result': {}}}: send a custom response
- {'return': {'error': {}}}: send a custom error

This way, a plugin can modify (/reimplement) or restrict the usage of
any of `lightningd`'s commands.

Changelog-Added: Plugin: A new plugin hook, `rpc_command` allows a plugin to take over `lightningd` for any RPC command.