Add setting for last-child float direction in grid

I think this setting is really usefull, it makes it possible to change the floating direction of the last-child element in grids.

Previously it used `$opposite-direction`:
```[class*="column"] + [class*="column"]:last-child { float: $opposite-direction; }```
Which can't be set to `left` if `$default-float` is already `left`.

It's also not possible to add something like
```[class*="column"] + [class*="column"]:last-child { float: left; }``` to you own css file, since this creates issues with centered columns where the float gets overridden. Thats why we need a setting!

I've set `$opposite-direction` as default, so it's fully compatible for older projects.