Story: Partial migration of data by entity types

[piotrekkaminski]

Currently the tool can migrate data all or nothing. But users are often willing to migrate only orders or only customers or catalog plus customers etc. See #497, #489, #337, #427, #326, #406, #80

Acceptance Criteria:

• ability to migrate data separately by its entity type: -- customers -- orders -- catalog -- CMS -- rules -- all other
• Partial migration of entities. For example: only last 1000 orders or orders were created for a specific period of date/time

:warning: Since there are many interconnections between entities (customers, orders, catalog ...) partial migration can break M2 store. For example if a developer migrate only orders and customers then merchant can potentially not able to process orders without existing products. This needs to be somehow accounted for not to introduce database inconsistency.

[diazwatson]

I have done this couple of times but I have to admit that it is always not an easy task I wonder what can be done to at least make things easier?