Fix YAML front-matter updating

[randycoulman]

When publishing a draft without a date, the code that inserts the publish date into the post was not restricting itself to the YAML front-matter section of the post. Rather, it would search anywhere in the post for the date: and title: keys and attempt to parse their values.

If a draft didn't have a date in the front matter, but had the pattern date: somewhere in the body of the post, read_post_yaml would find the pattern and attempt to parse the rest of the line.

If it was successful, it would then try to remove the old date entry and add a new one in the front matter, corrupting the body of the post.

I attempted to fix this by using YAML.safe_load to parse the front-matter, but ran into two issues:

1. It would parse the date as a date, causing breakage in other parts of the code that were expecting strings.
2. YAML.safe_load doesn't tell you where it finished parsing, so there was no way to get at the rest of the post after the front matter without using the regex anyway.

In the end, I opted for a simpler approach: use the regex (the same one that Jekyll uses) to split out the front matter and the body, update the front matter, and reassemble the new front matter with the existing body.

I was not able to craft a proper clash test for this fix; all my attempts to echo some additional content into the draft failed unless I ran with the -t flag. I'm not sure why that is.