postcss 5.x support

[pirxpilot]

Branch that supports postcss 5.x is here

Among other things it removes support for --safe option. And it requires node >= 0.12
Check it out, send feedback, patches etc. I plan to release it as postcss-cli 2.0.0 if everything works OK.

@FND - if you find a moment could you check if --watch works OK. Thanks!

[FND]

Yup, --watch still seems to work alright. However:

• postcss-import doesn't officially support PostCSS 5.x yet, so I had to manually remove that outdated package (i.e. rm -r node_modules/postcss-import/node_modules/postcss).¹ My project still seems to compile just fine afterwards.
• After doing so, make test-watch also succeeded, whereas before I just got some cryptic error.

¹ Thankfully, PostCSS emitted a helpful warning: "Your current PostCSS version is 5.0.1, but postcss-import uses 4.1.16. Perhaps this is the source of the error below."

[pirxpilot]

Thanks! Looks like postcss-import 5.x compatibility issues is now tracked: postcss/postcss-import#75
I'll add a workaround to Readme for now.