Always indent with only 2 spaces except when function arguments span

multiple lines.

* indent with 2 spaces

```
Enum.map(list, fn outer ->
  Enum.map(outer, fn inner ->
    some()
    code()
  end)
end)
```

* exceptions

```
function(arg1, arg2,
         arg3)
```

Author: tonini

elixir-smie.el

@@ -275,6 +275,12 @@
         (t
          (smie-rule-parent elixir-smie-indent-basic))))))
 
+    (`(:before . "fn")
+     (smie-rule-parent))
+
+    (`(:before . "end")
+     (smie-rule-parent))
+
     ;; Closing paren on the other line
     (`(:before . "(")
      (smie-rule-parent))

test/elixir-mode-indentation-test.el

@@ -572,24 +572,25 @@ x = do_something(
   :bar
 )")
 
-(elixir-def-indentation-test indent-inside-parens/2 ()
-  "
+(elixir-def-indentation-test indent-inside-parens/2
+    (:tags '(indentation))
+"
 x = do_something(:foo,
-                 :bar)"
-    "
+                    :bar)"
+"
 x = do_something(:foo,
                  :bar)")
 
 (elixir-def-indentation-test indent-inside-parens/3
-    (:expected-result :failed)
-  "
+			     (:tags '(indentation))
+"
 x = do_something(:foo, fn (arg) ->
                          do_another(arg)
                        end)"
 "
 x = do_something(:foo, fn (arg) ->
-                         do_another(arg)
-                       end)")
+  do_another(arg)
+end)")
 
 (elixir-def-indentation-test indent-lone-keyword ()
   "