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

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
~~~
