Custom counters for the import

[H3nSte1n]

Description:
At the moment there is only one counter. When a record has been successfully processed, the counter should be increased by one. There are more and more use cases where we need two or more counters.

Task
Add an option to define custom counters.
It could be used like add_counter :skips, :inserts, :shops.
=> After that you can use the increment_skips method to increment the skips counter.
=> different counter should be stored in meta
=> each counter will be resetted on every run