fix: Title not being extracted when not on the first line

While a page with a macro is processed, having a macro leaves an unclean buffer
for further processing steps, including title extraction.  The problem seems to
come from `ExtractMeta`: as it runs through the metadata, it hits the first
line of the macro but since it doesn't look like a complete Metadata (e.g.
matching either `\[\]:\s*#\s*\(([^:]+):\s*(.*)\)` or
`<!--\s*([^:]+):\s*(.*)\s*-->` in
https://github.com/kovetskiy/mark/blob/master/pkg/mark/meta.go#L37:L38, it will
break the loop in
https://github.com/kovetskiy/mark/blob/master/pkg/mark/meta.go#L61 and return
the final document as:
```
  Template: ac:children
  Style: h2
  Excerpt: none
  Page:
  TestSpace:Test
  Reverse: false
  All: true -->

:children:
```

This then goes into `ExtractDocumentLeadingH1` which doesn't find a match for
the regex `^#[^#]\s*(.*)\s*\n`, returning an empty title since the title is not
on the first line. This commit, while it doesn't fix the unclean document,
fixes the regex to properly find the title.

Closes kovetskiy#214

pkg/mark/markdown.go

@@ -345,7 +345,7 @@ func DropDocumentLeadingH1(
 
 // ExtractDocumentLeadingH1 will extract leading H1 heading
 func ExtractDocumentLeadingH1(markdown []byte) string {
-	h1 := regexp.MustCompile(`^#[^#]\s*(.*)\s*\n`)
+	h1 := regexp.MustCompile(`#[^#]\s*(.*)\s*\n`)
 	groups := h1.FindSubmatch(markdown)
 	if groups == nil {
 		return ""

pkg/mark/testdata/header.md

@@ -1,3 +1,4 @@
+
 # a
 ## b
 ### c