This introduces support for postprocessors that are run on the result of
a note that is being embedded into another note. This differs from the
existing postprocessors (which remain unchanged) that run once all
embeds have been processed and merged with the final note.
These "embed postprocessors" may be set through the new
`Exporter::add_embed_postprocessor` method.
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.
