Only enable wrapAround when slides fill carousel

Test case: https://codepen.io/desandro/pen/xJQwyV

Addressing #95. If you enable `wrapAround`, but the slides do not fill the carousel, you'll see a bug where the slides will jump around trying to fill the container. One solution is only enable `wrapAround` if there's enough content to fill the carousel, so this bug does not occur. This feature could be built into Flickity itself, with something like `wrapAround: 'fill'`.

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Start with the CodePen reproduction and issue #95, then trace Flickity's existing `wrapAround` behavior. Confirm the carousel no longer jumps when slides do not fill the container, while wrap-around still works when the content is sufficient; clarify whether the requested API is `wrapAround: 'fill'`.

Written by the indexing model from the issue text.