plugin/reporter.py: use unicode instead of str

I have a `'ascii' codec can't encode…` when a scenario fails.
I'm French and use accented steps. Maybe that's the reason.
This fix works for me.
Update README.md
Merge pull request #445 from Grahack/patch-4

plugin/reporter.py: use 'unicode' instead of 'str'
Support _ in update step property names

Allows us to specify fields like user_id or user__username when updating
steps.

Also update the documentation to be more correct and include an example.
Merge pull request #454 from infoxchange/fix-update-step

Support _ in update step property names