Add utl/logistic: maps 1st number x on each input line to logistic(x)

This is a little utility to map linear regressor prediction with --loss_function logistic into the actual logistic range.

It supports both [0, 1] and [-1,1] ranges.  Leaves rest of input unchanged.
Can be used on files or as a filter/pipe.

Ideally these should be implemented inside vw when predicting using a logistic model, but in the meantime this utility is easy to pipe the predictions through.