claytext: PARAGRAPH_RULES -> PARAGRAPH_LINE_FILTERS

Rename PARAGRAPH_RULES so that we can have other kinds of rules that act
on the paragraph as a whole, without the restriction for matching each
line.  Clarify its purpose in the comment.
claytext: introduce a block filter for :olitem

Split :listitem into two categories, :ulitem and :olitem.  Use
PARAGRAPH_BLOCK_RULES to parse the entire paragraph as a whole and
detect :olitem.