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.
It's possible to end up with "recursive embeds" when two notes embed
each other. This happens for example when a `Note A.md` contains
`![[Note B]]` but `Note B.md` also contains `![[Note A]]`.
By default, this will trigger an error and display the chain of notes
which caused the recursion.
Using the new `--no-recursive-embeds`, if a note is encountered for a
second time while processing the original note, rather than embedding it
again a link to the note is inserted instead to break the cycle.
See also: https://github.com/zoni/obsidian-export/issues/1
This refactors the Context to maintain a list of all the files which
have been processed so far in a chain of embeds. This information is
then used to print a more helpful error message to users of the CLI when
RecursionLimitExceeded is returned.
