Escape frontmatter content in raw records so it handles newlines

[lukewestby]

Hey Dillon!

This change escapes frontmatter content with JSON.stringify so that it handles newlines correctly. Without using JSON.stringify, newlines are included directly in the output which caus parsing failures. The difference looks like this:

Before:

"""{\"example\": \"hello\nworld\"}
"""

After:

"{\"example\": \"hello\\nworld\"}"

[dillonkearns]

Thank you so much Luke, this is great! I just encountered a similar problem with escaped quotes within frontmatter, and this solves that issue as well 🎉

[dillonkearns]

Just pushed out a release for this 👍 https://github.com/dillonkearns/elm-pages/blob/master/CHANGELOG-NPM.md#117---2020-01-12

Also, I've started adding contributors to the readme. But if you prefer not to, just let me know on the PR that it opens up and I'll close it, totally up to you 😄

@all-contributors please add @lukewestby for code.

[allcontributors]

@dillonkearns

I've put up a pull request to add @lukewestby! 🎉