fix: Correctly detect frontmatter to avoid mangling document

The previous format would incorrectly capture:
- A single horizontal rule at the top of the file
- Three literal dashes with arbitrary content after them.
This was detected as a frontmatter, and an extra HR would be inserted after as a result.

The new format requires that there are two sets of triple dashes on their own lines, where the first is the first text in the document.

lib/insert.js

@@ -28,7 +28,7 @@ module.exports = function insert(str, options) {
   if (m) newlines = m[0];
 
   // does the file have front-matter?
-  if (/^---/.test(str)) {
+  if (/^---\n.*\n---\n/.test(str)) {
     // extract it temporarily so the syntax
     // doesn't get mistaken for a heading
     obj = utils.matter(str);

test/test.js

@@ -426,4 +426,9 @@ describe('toc.insert', function() {
     assert.equal(strip(toc.insert(str, { linkify: true })), read('test/expected/insert.md'));
     assert.equal(strip(toc.insert(str, { linkify: false })), read('test/expected/insert-no-links.md'));
   });
+
+  it('should not mangle a file with an initial horizontal rule', function() {
+    assert.equal(toc.insert('---\nExample\n'),  '---\nExample\n');
+  })
+
 });