data-if attribute to conditionally remove element from DOM

[LeaVerou]

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[LeaVerou]

As discussed yesterday with @karger, documenting here for posterity:

Problem 1: If properties are inside a false if, do we save them?
Resolution: YES. This way if can be used for things like filtering and pagination without data loss.

Problem 2: Currently, multiple properties with the same name create an immutable collection. However, this might be confusing with structures like the following:

<div mv-if="...">
    <div property="foo" ></div>
</div>
<div mv-if="...">
    <div property="foo" ></div>
</div>

where the expectation is 1 property. However, in the general case, both ifs could be true, so we basically have something that could switch from a collection to a singleton every time the conditions recompute but is still saved as a collection due to the resolution above.

Implementation-wise, probably the easiest course of action is to keep this as a collection, since it's saved as one, and make the change that getData() on immutable collections returns a singleton instead of an array with one element.