minirst: support passing admonitions into findadmonitions() and parse() This will allow consumers to declare a custom list of admonitions to parse. Without this patch, custom admonitions would get removed when prunecomments() is run. We could add an argument controlling whether prunecomments() is run. However, it is better to convert the "paragraph" block to an "admonition" block so consumers don't have to parse for custom admonitions.