New progress bar for unknown amount of iterations in advance

[farchy]

Added a new progress bar for infinite (unknown max) iterations.
Added a test for it in test_progress.py

• edited relevant README.rst section

[verigak]

Hi, this is very interesting, but the implementation can be simplified a lot. You can implement an AdaptiveMixin that overrides percent and progress. Then AdaptiveBar can become a one liner:

class AdaptiveBar(AdaptiveMixin, IncrementalBar): pass

That way you can make any of the existing bars become adaptive.

[farchy]

ACK, will change the implementation...
Would you prefer that as a new PR or keep working on this?

[verigak]

Let's keep working on this one for now and see if we can squash the commits in the end.