Expand API

[nathonius]

I love the API, it's super useful in speeding up plugin development and keeping my code nice and tidy, while I let you do the hard work. 😉

However, there are a few gaps I think could be filled in the public API:

• deleteProperty - this would be nice to have along with add/update
• isYamlEmpty or removeYamlIfEmpty - this would be great to have to remove the frontmatter block entirely if the last property is removed
• addOrUpdateProperty - This certainly isn't hard to do with the add and update methods but I feel like this is common enough that it might make sense to have it as part of the public api

These are all methods I've ended up needing to create myself (or just copy from your source 😬).