Support :args: directive before method

[aycabta]

The :args: directive is supported only by after def keyword line like below:

  def method_with_args_comment(*array) # :args: a, b, c
  end
  # => documented as method_with_args_comment(a, b, c)

This commit changes to support like below:

  ##
  # :args: a, b, c
  def meth(*array)
  end
  # => documented as meth(a, b, c)

This fixes #433.

[hsbt]

New tests is fine to me.