Auto display

Add support for auto-display progress bar

This would resolve #4

A couple notes:
* this adds a dependency on `stm` and `stm-chans`, which should be
  pretty uncontroversial.
* I don't take a starting position in the auto-display form. It would be
  trivial to add but I think the aim with that function is to cover the
  common case, which is to start progress at 0 and have it manage
  displaying the progress.
* Added an example to examples.hs to show the usage.
* I would have added stuff to the README but it is blank, so that's a
  topic for another pull request ;)

Please let me know if you have any issues or things you'd like changed
with this PR.