Pystandups

* Fix 10: Will no longer check version at init to avoid requiring Python 3.7
* Require Python 3.6.1 because `python-prompt-toolkit` does: https://github.com/prompt-toolkit/python-prompt-toolkit/blob/5aea076692a304ec2bf8ad18fc59d4885cc462b1/setup.py#L32
* Require less buggy `openeditor` (see https://github.com/metov/openeditor/issues/6)

* Constrain `openeditor` version since <0.3 were buggy
* Make temp file for "done" be called `done` not `later`
* Use Markdown extension for temp files to encourage highlighting. Some editors syntax highlight based on extension and Markdown seems like a sane default.

* Add openeditor dependency (8)

* fix incorrect dependency

* bump version

Now using a different library or opening an editor. The new lib is simpler and more maintainable.

Replaced references to "yesterday" with "last standup". The program behavior doesn't change, except that the CLI args are slightly different now.