m-attr-lerp #165

[MarcusLongmuir]

Resolves #165

This PR adds a new element, m-attr-lerp that allows smoothly lerping between element attribute values similarly to m-attr-anim, but without having to specify document-time based timings and explicit start and end attributes.

Example usage:

<m-cube x="-3" onclick="this.setAttribute('x','3');">
  <m-attr-lerp attr="x" duration="2000" easing="easeInOutCubic"></m-attr-lerp>
</m-cube>

The "attr" attribute also allows specifying multiple comma-separated attributes or all to apply to all attributes at once (which is also the default if attr is not provided).

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What kind of changes does your PR introduce? (check at least one)

• [x] Feature

Does your PR introduce a breaking change? (check one)

• [x] No

Does your PR fulfill the following requirements?

• [x] All tests are passing
• [x] The title references the corresponding issue # (if relevant)