Improve performance of control comments in postcss-custom-properties

[romainmenke]

closes : #730

@marvinhagemeister After discussing this with @Antonio-Laguna we decided to keep the functionality but to improve performance sufficiently.

We found that the performance issue only happens in fairly large code bases with large numbers of custom properties.

This change has three parts :

• more efficient way to gather and parse custom properties from html/:root elements
• more efficient way to check if a control comment exists
• caching the existence of control comments in a WeakMap.

With these changes the overhead of the feature becomes negligible.
O(n^2) -> O(n)

[Antonio-Laguna]

This does look good to me!

Thanks!