Update changelog for 0.1.8
Update changelog for 0.1.8

ulrikaugustsson/mdast,eush77/remark,ulrikaugustsson/mdast,yukkurisinai/mdast,tanzania82/remarks,yukkurisinai/mdast,chcokr/mdast,chcokr/mdast,eush77/remark,wooorm/remark