0.2.0 new flag `--skip-existed` & check for already existed variables & connections

1. Added check for variables - now if variable already exists on server Airflow Helper will raise error if you tries to overwrite it from the config.
   To overwrite existed Variables, Connections, Pools - use flag '--overwrite' or argument with same name, if you use Airflow Helper from Python.

2. Added flag --skip-existed to avoid raise error if variables/connections/pools exists already on Airflow Server - it will just add new one from config file.