xlim accept date/datetime strings, and/or date/datetime objects

[DanielGoldfarb]

• Prior to this version, xlim kwarg accepted only float or int:
  • float as matplotlib date; (only when show_nontrading=True)
  • int or float as dataframe row number; (only when show_nontrading=False)
• xlim kwarg now also accepts
  • date or datetime string
  • date or datetime object (datetime.datetime or pandas.Timestamp)