lex.go: Add isComment()
lex.go: Add lexComment()
node.go: Add NodeComment
parse.go: Add sectionLevel.overLine

Refactor sectionLevels.Add() to compensate for differing overlines between
sections with similar adornment runes. If two sections with similar adornment
runes are encountered, but differing overlines, they are not of the same
section level.
Add secction test OverlineBad0500

Tests for section overlines that begin with ".." which is a comment token.
parse_test: Add spaces before Got: in test output