function getDirections(orientation, defaultLayout) {

  // don't override specific trbl directions
  if (isExplicitDirections(defaultLayout)) {
    return defaultLayout;
  }

  switch (orientation) {
  case 'intersect':
    return 't:t';

  case 'top':
  case 'bottom':
    return 'v:v';

  case 'left':
  case 'right':
    return 'h:h';

  // 'top-left'
  // 'top-right'
  // 'bottom-left'
  // 'bottom-right'
  default:
    return defaultLayout;
  }
}
Returns the manhattan directions connecting two rectangles
with the given orientation.

Will always return the default layout, if it is specific
regarding sides already (trbl).

@example

```javascript
getDirections('top'); // -> 'v:v'
getDirections('intersect'); // -> 't:t'

getDirections('top-right', 'v:h'); // -> 'v:h'
getDirections('top-right', 'h:h'); // -> 'h:h'
```

@param {string} orientation
@param {string} defaultLayout

@return {string}