fix($markdown): sidebar headers not being detected (close: #841)

header will not be detected when some content are placed before h1

lib/util/index.js

@@ -34,7 +34,7 @@ exports.inferTitle = function (frontmatter) {
   if (frontmatter.data.title) {
     return deeplyParseHeaders(frontmatter.data.title)
   }
-  const match = frontmatter.content.trim().match(/^#+\s+(.*)/)
+  const match = frontmatter.content.trim().match(/^#+\s+(.*)/m)
   if (match) {
     return deeplyParseHeaders(match[1])
   }