Optimize Parser; Fix Off-by-One Errors & Related Test Cases

; ATX Headers
: Do not include leading or trailing spaces on the line
  in the output text.

; Paragraph Ending
: Fix a line of paragraph text from disappearing when
  the last paragraph in the input ends without any input.

; Setext H1 by Itself on a Line
: These equal signs should not produce any output.

TODO: Introduce actual ATX header? At this rate, the equal
signs are really ambiguous.