Provide a single export command for all entity types

[ademarco]

We could refactor our CLI in order to have only one command for all entity types and not one command per entity type. For example, instead of having to run:

$ drupal content-sync:export:nodes ../content/
$ drupal content-sync:export:taxonomy-terms ../content/
etc.

We would run:

drupal content-sync:export --entity-type=node ../content/
drupal content-sync:export --entity-type=taxonomy_term ../content/