Current indent + 2 is a valid stop (fix #250)

Fixes #250.

This would lead code bodies in `data`, `instance` and `mutual` blocks to a 2-spaces indentation with `C-j`, which is much nicer than the current indent style.

instead of:

```idr
mutual
        even : Nat -> Bool
        odd : Nat -> Bool
```

We now have:

```idr
mutual
  even : Nat -> Bool
  odd : Nat -> Bool
```

(Note: this is a different behavior from [haskell-simple-indent.el](https://github.com/haskell/haskell-mode/blob/master/haskell-simple-indent.el). Personally I always prefer `haskell-indentation`, for `simple-indent` does not indent code blocks in the way I wanted. I'd like to see if someone can bring [idris-indentation](https://github.com/idris-hackers/idris-mode/blob/87ee0433addfa3b49ac15857fb81f82c2bbdb4cc/idris-indentation.el) back, but for now, this workaround fixes the indentation I had trouble with.)