Support indentation to continue a function's argument list

When I hit Tab with the cursor at <> below:

```
return foo
<>
```

I'd like the cursor to go here:

```
return foo
<>
```

And if a $ was present, then:

```
return $ foo
<>
```

I tried making the change myself in haskell-indentation.el, but I couldn't understand how the parser works. If you can tell me where to look, I'd be happy to try.