text: Title: Consider allowing PythonCallable initialization with dotted paths
  Body: We're currently working on exporting DAGs to Airflow but PythonCallables won't be supported for the first version. When we convert a Ploomber DAG to Airflow, we need to instantiate the Ploomber DAG, if it has PythonCallables, the actual functions have to be imported, which implies that all imported packages in those functions must be installed in the Airflow host.
  
  This is highly problematic, as it can pollute the python installation where Airflow is running and it requires all dependencies to be installed before parsing the DAG.
  
  A possible solution would be to let PythonCallables be initialized with dotted paths, so they don't have to import the source function until the task is executed.