Mithril.js: Allow CSS vars to be specified via `style` attribute

[barneycarroll]

Steps to Reproduce

In this example…

Expected Behavior

The first line of buttons should appear the same as the second.

Current Behavior

The first line of buttons do not have the CSS variable declarations applied, because CSS variables must be declared via setProperty

Possible Solution

Use the explicit style.getPropertyValue & style.setProperty APIs instead of treating style as a POJO to be assigned to

[barneycarroll]

Was worried about the potential perf considerations of this but it turns out React implemented this a year ago and it improved perf

[isiahmeadows]

@barneycarroll If that's the case, then let's do this.

[barneycarroll]

Wilco, I'll get on it in a tick. This will make interoperability with framework agnostic CSS-in-JS patterns far more powerful!