Fix support for nested emphasis #398
Description
Subject of the issue
Asterisk parsing is incorrect for this case:
*foo ***bar*** baz*
Steps to reproduce
var remarkParse = require("remark-parse")
var unified = require('unified');
const parser = unified().use(remarkParse);
console.log(parser.parse('*foo ***bar*** baz*'));
Expected behaviour
Returned node doesn't contain an asterisk text.
Actual behaviour
