Netlify-cms: Support custom delimiters for frontmatter.

[tech4him1]

- Do you want to request a feature or report a bug?
feature

- What is the current behavior?
After https://github.com/netlify/netlify-cms/issues/563, we now support frontmatter in multiple formats, but we only allow it to be parsed using the default delimiters.

- What is the expected behavior?
It would be nice to allow the user to specify their own delimiters for parsing frontmatter out of an entry file. @erquhart suggested using frontmatter_delimiter: "~~~".

[erquhart]

Note: could also accept an array of opening/closing delims, as gray-matter does: ['~~~', '~~~']. No, I cannot think of a use case for this lol. But I'm sure someone can.

[Swieckowski]

I'd like to take this, but it might take me a bit since I'm green to the codebase.