Support multiple fields for -uri_id

[RobertSzkutak]

It would be convenient if you could set -uri_id to multiple fields that when concatenated create a unique ID.

For example:

For incoming data:

part_number,manufacturer_id
1234,ABCD

Running ...

-uri_id part_number,manufacturer_id
-output_uri_suffix .json

Could result in:

1234/ABCD.json

[josvanroosmalen]

Why does this take so long to implement this 5 minute change? I would also to propose to introduce uri_seperator. So if uri_separator = '-' then above example become 1234-ABC