Fix quadratic list indexing in commonmark/man/plaintext output formats

[kevinbackhouse]

Fixes: #321

This adds some code to cmark_render() to calculate the correct list item index and store it on the node, so that output formats like commonmark don't have to calculate it themselves.

Nodes of type CMARK_NODE_ITEM store the same kind of data as CMARK_NODE_LIST, as can be seen here:

cmark-gfm/src/blocks.c

Lines 1371 to 1376 in c4de252

	// add the list item
	*container = add_child(parser, *container, CMARK_NODE_ITEM,
	parser->first_nonspace + 1);
	/* TODO: static */
	memcpy(&((*container)->as.list), data, sizeof(*data));
	parser->mem->free(data);

So I decided to use the start field to store the index.

[gravypod]

@kevinbackhouse, for what it's worth this passes our internal test suite. I am not a huge fan of how list numbers are calculated in the main render loop, and not at parse time, but I think that can be improved in future PRs.