Fix formatting breaking markdown frontmatter

[Skadic]

This should fix #10. Using this markdown file as an example:

---
item1: hello
item:
- my
- list
---

# Title

This is sentence one. This is sentence two.

Previously, the output from mdslw was:

---
item1:
hello item:
- my
- list
---

# Title

This is sentence one.
This is sentence two.

Note, that the frontmatter is broken now. My code changes strips the frontmatter from the markdown input before formatting, and adds it back at the end. Now, running mdslw results in:

---
item1: hello 
item:
- my
- list
---

# Title

This is sentence one.
This is sentence two.

[razziel89]

Thanks again! The fix including some adjustments can be found in PR #13. Closing in favour of #13.