Rename Pos type to "Line"
lex.go: Add StartPosition type
lex.go: Rename l.pos to l.index
lex.go: go fmt fixes
node.go: Rewrite

Add Node types implementing the Node interface. Most notably SectionNode.
parse.go: Rename Root to Document
parse.go: Add error methods to the Tree type
parse.go: Begin implementing item type switch in t.parse

Add t.section to check section item correctness.
parse.go: Add functions for controlling the parser
parse_test.go: Add parseTest()