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.
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