Allow indentation of multiline function arguments to indent to the first argument of the first line

[ckruse]

So that code like this

defmodule Foo do
  def foo() do
    my_fun(a, b, c,
      d, e, f)
  end
end

could also be indented like this:

defmodule Foo do
  def foo() do
    my_fun(a, b, c,
           d, e, f)
  end
end