Frontmatter Support

Is your feature request related to a problem? Please describe. Frontmatter is wide used standard to add metadata usually in YAML format to markdown files. It is used in a variety of Static Site Generators like Jekill and Hugo. The format is very simple as a put the YAML between '---' in the init of de markdown file. But, for use it with dasel, first I've to extract the YAML with sed/awk before. And rewrite the header become very hard.

Describe the solution you'd like I would like that dasel supports frontmatter.

Describe alternatives you've considered To use it now I have to extract the from the header.

Additional context https://jekyllrb.com/docs/front-matter/