Improve train CLI with base model (#4911)
Improve train CLI with base model (#4911)

Improve train CLI with a provided base model so that you can:

* add a new component
* extend an existing component
* replace an existing component

When the final model and best model are saved, reenable any disabled
components and merge the meta information to include the full pipeline
and accuracy information for all components in the base model plus the
newly added components if needed.
honnibal/spaCy,explosion/spaCy,spacy-io/spaCy,spacy-io/spaCy,explosion/spaCy,explosion/spaCy,explosion/spaCy,spacy-io/spaCy,spacy-io/spaCy,honnibal/spaCy,spacy-io/spaCy,spacy-io/spaCy,honnibal/spaCy,explosion/spaCy,explosion/spaCy,honnibal/spaCy