VIS: plot with `use_index=False` shouldn't plot the index name

Add test case

Add check for use_index before setting label

Add to release notes

Add test case for barplot

Add checks for use_index in bar plot

Missed merge conflict marker
Merge pull request #8812 from onesandzeroes/plotlabel

VIS: plot with `use_index=False` shouldn't plot the index name
Add test for unicode separator and data
ENH: Allow unicode separators in to_csv & read_csv

* unicode delimiters coercible to ASCII (e.g., `u(',')`) are always allowed.
* unicode delimiters that don't fit within ASCII are always disallowed
  with C engine.
* unicode delimiters that don't fit within ASCII will never work in
  Python 2 engines (because their CSV assumes seps are unicode).