Add escape hatch

`prettier` at this point is still outputting invalid JavaScript or badly formatted output in many cases. This will go down as time goes but in the meantime, it would be great to be able to disable it for just an expression.

Proposal:

```js
// prettier-ignore-next-block
function a() { 
  iCanDoWhateverIdent
    (a, b,); }
```

and prettier will keep the next AST node as is.