timesketch_importer reuse existing sketch by name

[cvandeplas]

Is your feature request related to a problem? Please describe.
Currently, with timesketch_importer.py there are two ways to define the sketch:

1. reuse existing sketch by giving sketch_id
2. create new sketch by giving sketch_name

Describe the solution you'd like
Reuse existing sketch by giving sketch_name

Additional context
A pull request this minor change is coming in a moment