Use wcwidth to handle fullwidth characters

[JeroenDelcour]

tplot currently assumes all characters are halfwidth. We could handle fullwidth characters by using wcwidth to get the displayed width of each character and then removing spaces from that row as needed to maintain alignment with the other rows.

I suspect this would have to be done in the __str__ method after converting the self._canvas array to a list of strings, since it will mean the canvas is no longer rectangular.