Provide optional printout of progress

[tibshirani]

trafficstars

I ran a regression_forest for > 10 minutes and had no idea if it would complete in 15 min or an hour.

It would be great to have an argument "verbose" (default FALSE) which causes the function to print the function's progress, to help the user estimate the remaining time before completion.

[ginward]

This feature looks important to me, too. Especially when training the forest on large datasets.

[ginward]

It will be nice if it could print out that how many trees have been grown v.s. how many trees are left to grow.

[erikcs]

For reference ranger does this here

https://github.com/imbs-hl/ranger/blob/master/src/Forest.cpp#L1072