Add option to enable/disable auto margin on carousels

[battaglr]

By default all items on a carousel add a margin depending on the space available. But since we can't control that margin, some times we can't meet certain design needs.

To fix that I added an option to enable/disable this behaviour, leaving it enabled by default.

Here is a test case to show the new functionality.

Thanks to @fambrosi for the help!

[atma]

@battaglr Would be nice to have this o similar test case in the views for development, really very handy.

[battaglr]

@atma, awesome! It's a good idea, I'll add something simpler —using just an <img />— in another PR. I'll also open an issue so I don't forget.