Instead of passing clones of context and the markdown tree to
postprocessors, pass them a mutable reference which may be modified
in-place.
This is a breaking change to the postprocessor implementation, changing
both the input arguments as well as the return value:
```diff
- dyn Fn(Context, MarkdownEvents) -> (Context, MarkdownEvents, PostprocessorResult) + Send + Sync;
+ dyn Fn(&mut Context, &mut MarkdownEvents) -> PostprocessorResult + Send + Sync;
```
With this change the postprocessor API becomes a little more ergonomic
to use however, especially making the intent around return statements more clear.
This change introduces a new `--hard-linebreaks` CLI argument. When
used, this converts soft line breaks to hard line breaks, mimicking
Obsidian's "Strict line breaks" setting.
Implementation detail: I considered naming this flag
`--strict-line-breaks` to be consistent with Obsidian itself, however I
feel the name is somewhat misleading and ill-chosen.
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.
