node.go: Don't set Level on SectionNode init
parse.go: Remove sectionLevel struct
parse.go: Update sectionLevels.String()
parse.go: Refactor sectionLevels.Add()
parse.go: Add Tree.nodeTarget
parse.go: Remove Tree.sectionLevel
parse.go: Rewrite section level handling
parse_test.go: Update TestSectionLevels* tests
parse_test.go: Remove \n from error output
Fix #parse-tree for SectionLevelTest1