src/language-markdown/parser-markdown.js/splitText
function splitText() {
  return ast =>
    map(ast, (node, index, parentNode) => {
      if (node.type !== "text") {
        return node;
      }

      let value = node.value;

      if (parentNode.type === "paragraph") {
        if (index === 0) {
          value = value.trimLeft();
        }
        if (index === parentNode.children.length - 1) {
          value = value.trimRight();
        }
      }

      return {
        type: "sentence",
        position: node.position,
        children: util.splitText(value)
      };
    });
}