When you set debug_mode, logger output stdout.
Implement BlackbirdError object.

* When an error occurs in your plugins, you should raise this error.
Implement BlackbirdError

* Changed error object name BlackbirdError -> BlackbirdPluginError
* Implement BlackbirdError
Implemente LLD thread.

If plugin module has "build_discovery_items" method,
blackbird create new thread for LLD and starts this.
Update changelog
Following commits

* Executor catch BlackbirdPluginError
* fix typo
    + creater -> creator
    + executer -> executor