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)
```
update CHANGELOG
Merge pull request #149 from elixir-lang/indent-inside-parens

Always indent with only 2 spaces except when function arguments span multiple lines.