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.
Nick Groenen