Use a different cost function to select the best paragraph

[gucci-on-fleek]

Right now, lua-widow-control selects the paragraph with the fewest absolute demerits. However, a paragraph's demerits is the sum of the demerits for each line, so a longer paragraph will almost always have more demerits, even if it has a lower badness.

Choices

cost = demerits

This is what we have right now. This probably isn't a great choice, for the reasons discussed above.

cost = demerits / prevgraf

This is a decent option. It tries to minimize the average demerits for each line.

cost = sqrt(demerits) / prevgraf

This tries to minimize the badness for each line.

cost = demerits / sqrt(prevgraf)

This tries to minimize the average demerits, while slightly penalizing longer paragraphs. This makes sense, since given two paragraphs with equal average demerits per line, you should prefer the shorter one since it will give fewer overall "bad" lines.