Ruby AST: Extract Node#header and use it

This puts processing of |if|, |unless|, |case|, |while| and |until|
headers at one place, which will be useful when I'll change it.
Ruby AST: Improve emitting of headers (for |if|, |unless|, etc.)

Before:

  if some_long_expression &&
    another_long_expression
    do_something
  end

  if method(
    some_long_argument,
    another_long_argument
  )
    do_something
  end

After:

  if some_long_expression &&
      another_long_expression
    do_something
  end

  if method(
      some_long_argument,
      another_long_argument
    )
    do_something
  end

Similarly for |unless|, |case|, |while| and |until|.