lex.go: Add pos argument to lex.backup()
Misc doc improvements
lex.go: Rewrite isStartOfLine()
lex_test.go: Change error output flags back to %#v
node.go: Add NodeIndent
parse.go: Remove peekCount from peek()

Using peekCount instead of just checking for nil is too error prone.
parse.go: Add itemEOF back to Tree.parse() switch
parse.go: Remove pos argument from Tree.peekSkip()

peekSkip() is mostly used to look ahead past a particular item for the very
next token. Having an argument for more positions past that made the function
too complicated.
parse.go: Remove peekCount from Tree.next()

Check for nil instead, peekCount complicated matters.
Add section test 009

Tests for inset section titles with overline.