• Added strikethrough and italic to Styler
• Raising TypeError in to_excel if trying to use a non-openpyxl engine
• Exposing Exposing StyleFrame.ExcelWriter and StyleFrame.read_excel to the module-level (styleframe.ExcelWriter
  and styleframe.read_excel) to more closely mimic pandas' API
• Fixes GitHub issue #124 - Improved calculation of color luminosity from tint