Support integer passthrough when hashing strings for dsjson and vwtest

[jackgerrits]

Users of these two formats expect to be able to (via a config option) treats strings which are just integers as the integers themselves when they are "hashed", rather than being actually hashed.

This should be able to be configured, so the question is is this runtime config or model config? I am pretty confident it is runtime config. Additionally, since it does not apply to all parsers the question of parser specific configuration comes up too.

In the new Json format the proposal for the same behavior could be to extend the usage of the : special char and make any number prepended with a : to be treated as the number itself and not hashed.