Add support for postprocessing of Markdown prior to writing converted
notes to disk.
Postprocessors may be used when making use of Obsidian export as a Rust
library to do the following:
1. Modify a note's `Context`, for example to change the destination
filename or update its Frontmatter.
2. Change a note's contents by altering `MarkdownEvents`.
3. Prevent later postprocessors from running or cause a note to be
skipped entirely.
Future releases of Obsidian export may come with built-in postprocessors
for users of the command-line tool to use, if general use-cases can be
identified.
For example, a future release might include functionality to make notes
more suitable for the Hugo static site generator. This functionality
would be implemented as a postprocessor that could be enabled through
command-line flags.
Previously, links referencing a heading (`[[note#heading]]`) would just
link to the file name without including an anchor in the link target.
Now, such references will include an appropriate `#anchor` attribute.
Note that neither the original Markdown specification, nor the more
recent CommonMark standard, specify how anchors should be constructed
for a given heading.
There are also some differences between the various Markdown rendering
implementations.
Obsidian-export uses the [slug] crate to generate anchors which should
be compatible with most implementations, however your mileage may vary.
(For example, GitHub may leave a trailing `-` on anchors when headings
end with a smiley. The slug library, and thus obsidian-export, will
avoid such dangling dashes).
[slug]: https://crates.io/crates/slug
