[dagstermill] Allow for tags & description to be passed into solids (issue 3290)
Summary: Change solids.py::define_dagstermill_solids to allow for custom descriptions & tags. Default tags & descriptions are the same as before, unless explicitly written over by user input. See https://github.com/dagster-io/dagster/issues/3290 for more context.
Test Plan: Added tests for custom/default tags & descriptions, respectively. For tags, checked that 1) default tags were present (didn't test overwriting them bc. I can't imagine a case where the user would want to change the "kind" : "ipynb" mapping for a notebook, for example). I also checked 2) that custom tags were present when added. For descriptions, I checked that the default description was present unless a custom description was passed in.
Reviewed By: catherinewu
Differential Revision: https://dagster.phacility.com/D6141