use models.Note in read operations

[jacobb]

For 0.4 and 0.5, we moved to using a Note struct to centralize a lot of the work with individual markdown files. Our read operations could probably benefit from this as well, as a way to centralize likely redundant calls of frontmatter/markdown parsing.

[jacobb]

#15 handles a lot of this, but I think it's still work reviewing to see if there's even more areas where we're making redundant calls