[Feature request] Modernize Slide Container using Flexbox

Currently, `fp-slide` and `fp-slidesContainer` rely on `float: left` and `display block`. I propose transitioning the slides to use `display: flex` flexbox.

One benefit is that on auto height section with slide, slides take up the height of max height slide and the rest would center horizontally.

```
.fp-slidesContainer {
height: 100%;
display: flex;
flex-direction: row;
...
}
```

That being said, I'm not sure of the corner cases and if this change would be backwards compatible though. I'm sure you've thought of this so wanted to get your opinion.

Thanks 🚀