Compute trailing depending on value.

Property definitions

markdown $ MDLine :: process_trailing
	# Compute `trailing` depending on `value`.
	fun process_trailing: Int do
		var count = 0
		var value = self.value
		while value[value.length - count - 1] == ' ' do
			count += 1
		end
		return count
	end
lib/markdown/markdown.nit:1489,2--1497,4