Vertical Ruler Support

In other editors that support vertical rulers, I like to have two rulers: one at 80 columns and another at 120 columns. This allows me to format my documents so that people reading them in fixed-sized terminal windows--or otherwise small screens--can read them without having to scroll left or right. I am requesting support for being able to add one or more vertical rulers to MacDown. Thank you.

The issue names no files, tests, or entry points. Start by locating MacDown's editor and settings entry points, then define how users configure one or more rulers; done means rulers can be enabled at positions such as 80 and 120 columns.

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