`droppable` attribute is not respected.

Docs mentions that `droppable` attribute defines `Whether the column is a valid drop target.`.
I tried setting it to `false` for a column but still it gets displaced by dropping another column on it.

I think we should have a check for `droppable` attribute after below line,
https://github.com/offirgolan/ember-light-table/blob/master/addon/mixins/draggable-column.js#L116
```javascript
if(targetColumn.droppable){
//then proceed
}
```
Created a PR for this #496