Add `markup` to the i3bar protocol as a block member.
This is a string that determines how the block should be parsed as
markup. "pango" indicates the block should be parsed as Pango markup.
"none" indicates the block should not be parsed as markup.
Parse text within workspace buttons and the i3bar statusline as Pango
markup. This lets people specify things like font weight, text color,
background color, font size, and font family in the text of i3bar.
fixes#1468