Added `paragraph` nodes to tests. Also dealt with whitespace parsing
ripple effects created by introduction of `paragraph`.

- Fixed text node consolidation
- Removed unecessary newline filter passes
- Traverse entire AST tree, including nested children when inserting
  newline nodes into text nodes

Newlines inside paragraphs will generate hard line breaks. This is a
departure from "standard" Markdown which requires an invisible delimiter
(two spaces followed by a newline) for the same behavior.

See recent greenbar_markdown commits for the rationale behind this decision.