[do not merge] Exercise: add outer margins

[kaelig]

Because o-grid is pretty complex, I built an exercise for you @AlbertoElias, so that you can understand the code of o-grid.

At the moment, outer margins (space on the left and right of a o-grid-container) are the same as gutters: 10px < M, and 20px on layouts >= M.

It works exactly like for gutters.

This branch contains a few tests. To make them pass here is what is needed:

• add an $o-grid-outer-margins map that contains outer margins for each breakpoint

• modify the mixin oGridAddLayout so that developers can add a layout with a specific $outer-margin-width:

  @mixin oGridAddLayout($layout-name, $layout-width, $gutter-width: null, $outer-margin-width: null) {

ps: not sure that should be merged when you're done, since it's not something anybody needed. But after doing this exercise you should know all the secrets behind the grid!

[kaelig]

Oops this is a PR onto the wrong branch. See #106 instead