Complete the modifiers on container and units of the grid

Hi Nicolas,
Currently, griddle allows us to define the alignment of the grid and the text in the units thanks to the same variable: `$grid-direction`.

However, no modifier can reverse that. We can center one or more units but we can't pull it/them on the right or on the left according to the cases.
I therefore propose to complete the modifiers to do that like `grid--center` or `grid__cell--center`.

We may use:

* `grid--right` and `grid__cell--right` when grid direction is set to the left.
* `grid--left` and `grid__cell--left` when grid direction is set to the right.

It is not a big feature, but this can prove to be useful in some cases.
I put online some test cases to get a result idea more easily: [see there](http://7studio.fr/github/griddle/feat-direction-modifiers/).

I hope you will find this pull request interesting,
Xavier.