Output HTML Checkbox

[KyleKing]

For consistency with the .js implementation (jgm/djot.js#102), it would be nice to output the HTML standard for input type=checkbox rather than the li with classes (https://github.com/sivukhin/godjot/blob/975b00d3ec5862ca6165c33272bb1bc2c1be8b9b/djot_parser/examples/todo-list.html)

I'm happy to submit a PR if this seems like a beneficial change. I'm not sure how changes are reflected across djot rendering implementations normally and this could be a breaking change for end users

[sivukhin]

Hi @KyleKing!

I think this is a good change and it should affect only rendering layer (see djot_html.go/DefaultConversionRegistry) which is exposed in godjot and can be redefined by user.

As godjot do not have a lot of users (I am actually not aware of any) - I think this change will be pretty safe and should not affect any users :)

So, feel free to open a PR!