Add support for strikeout/strikethrough/linethrough text

Adds support for text with a line through it throughout the stack. 

In Qt this format is called StrikeOut so I kept that naming convention throughout.

Added an optional 11th parameter to the lua function setTextFormat to control whether strikeout is used.
Created new lua function setStrikeOut that exposes strikeout functionality. Usage is just like setBold.

Will add cecho tag support in a subsequent commit.
Could you fix this one up as well?