Update postcss dependency and make it a peerDependency

[mariusGundersen]

the postcss dependency is very old, it should be updated to a newer major version. It should also be marked as a peerDependency so that the main project can decide on what version to actually use

[RyanZim]

The plugin code itself would also need updated to make this work with the latest postcss. PR welcome if somebody wants to update this plugin; I'm happy to review/publish.

[MoOx]

It seems that this plugin, which was mostly used in cssnext & cssnano v3, isn't really used anymore (most download comes from v2, v3 has almost no download). I will make publish this as the work is already done (took a few minutes), but I will add a notice about the goal of this plugin.