Feature: hard wrap lines to specified length

[afontenot]

There was an issue for this, if I'm reading it correctly, but it was closed because the issue filer now uses an IDE to do this.

I'd like a flag like -wrap <int> to set a maximum line length, with markdownfmt reflowing paragraphs automatically to fit the limit.

Something like the following logic should be appropriate:

• Never reflow literal environments (code blocks, tables, etc).
• Reflow quote environments to maintain the indentation of the first line at the current depth, and prefix continuing lines with "> ".
• Reflow bulleted / numbered environments by reflowing each item separately, maintaining the indentation of the opening text of the item on succeeding lines.
• Reflow all other environments, inserting a newline at a word boundary when adding the next word would exceed the character limit.
  • Always insert at least one word (or URL, image, or other entity) on each new line, even if that would cause the line to exceed the character limit.
  • Count characters using grapheme clustering, eg. with uniseg.