Custom title should also be used for the actual headline

When specifying a custom title with the `@vuepress` annotation, it should also be used in the generated markdown file.

Using:
```
/**
 * @vuepress
 * ---
 * title: Your custom title
 * ---
 */
```

Actual output:
```
---
title: Your custom title
---
# action 
```

Expected output:
```
---
title: Your custom title
---
# Your custom title 
```