Define cssnext as a postcss plugin internally

[MoOx]

We should follow cssnano api

• cssnext is defined as a postcss plugin (minor change)
• cssnext(options) return a postcss plugin (current behavior)
• cssnext(options).process(string) => result should be the implemented & promoted (new interface)
• maybe we can keep cssnext(string, options) api to avoid a major breaking change (current behavior) - not sure it's a good idea because postcss.plugin might add more change in the future. @ai any comment on that ?

This way, people might use directly cssnext (without braces) as a postcss plugin when postcss/postcss#402 is fixed.

Not a big deal, but I think the reusable instance is a good idea.

Depending if the current api (string, options) can be kept, we might push this as a minor change or as a major bump (2.x)

[ben-eb]

I wouldn't recommend overloading your main exported method. Embrace breaking changes. 👍

By the way, process API should look like cssnext.process(str, opts)

[ai]

Nice plan.

I think you should remove cssnext(cssnext, opts) API, because it makes code too complicated.

[MoOx]

You cannot say that var css = cssnext(string, options) is complicated versus cssnext(options).process(string).css or cssnext(options).process(string).then(result) { result.css ...}). That's not true ;)

[MoOx]

cssnext is now postcss-cssnext. cssnext is deprecated.