Add ability to use synchronous

Summary

I can't seem to find a way to use this in a sync way. I can't use it async because I want this to use with Storybook, which doesn't have a way to add docs with the notes addon in an async way. Looking at the source code, it's also not possible to just use the parser in a sync way after loading the file myself. If this would be changed, then the user can choose to use the parser only for example in a sync way.