Don't match front matter in the middle of the file

[mojavelinux]

The front matter parser is entirely too loose. The idea of front matter is that it's at the top of the file. The current implementation is looking for lines that look like front matter anywhere in the file.

The parser is also running .strip on the line before analyzing it, which causes it to match lines that have indented ---.

Both of these loose checks cause processing errors on Markdown (and other markup languages) that allow three hypens in the content.

The rules for locating front matter should be as follows:

1. The first line must start with three hyphens (e.g., ---), followed only by whitespace
2. The front matter ends when the next line that starts with three hyphens followed only be whitespace is found
3. No other lines in the file will be scanned for front matter delimiters outside of these two matches

[mojavelinux]

This was the cause of issue #335.