node.go: Remove NodeBlankLine

Blank lines are only kept if the text is in a literal block.
node.go: Remove SpaceNode

Indentation is only used to indicate blockquotes, definition lists, and local
content, such as indentation of a section title.
node.go: Remove StartPosition from Node

Some Nodes don't require a start position such as BlockQuoteNode.
node.go: Add missing NodeType constants
parse.go: Add indentation level tracking
parser.go: Add type parserError and severity levels
parser.go: Expand Tree literal
parse.go: Implement id handling

Since itemBlankLine and itemSpace are skipped over, the parser has to keep
track if the id of a Node.
go fmt fixes
parse.go: Add indent() and const indentWidth
parse.go: Increase width of t.token
Fix UnexpectedTitle test data
parse.go: Remove checks that result in panic
parse.go: Add Tree.skip()