Rename `pos_label_str` to `pos_label` and move to Input section

[desilinguist]

We should implement this in a backwards compatible way along with deprecation warnings in the next release and then fully deprecate the old behavior in the release after that.

[desilinguist]

I think a good compromise is to change the name to pos_label but keep it in the Tuning section so that the changes that folks have to made to their old config files are minimized and can be done in an automated fashion.