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.
