[WIP] Use postcss-cssnext

[jednano]

Based on #219.

This is just a work in progress for now.

[jednano]

@MoOx it looks like the async issue is the only thing failing the build at this time. Also, maybe we should move the features benchmarks to postcss-cssnext? What do you think?

[MoOx]

The async issue is a big deal because all existing runners are using it, so when we will change the API by removing the simple cssnext(string, options) in favor of the postcss style cssnext(options).process(string), we will loose compat with all runners. That why I planned to deprecate them (see #208 (comment)) first (and communicate about it) before upgrading cssnext itself (or not? to deprecate it directly?).

[jednano]

Totally makes sense.