Add "input" and "output" option to config file

[Gerhut]

It will make npm script written more conveniently without specify input and output files in command line.

[MoOx]

Will try to add that the next time I will work on the cli.

[Gerhut]

Can I try implementing it and send a PR?

[MoOx]

Not a good idea. The cli will be rewritten at any moment (see #208 + wip of https://travis-ci.org/MoOx/cli-for-files) so please wait for this.

[MoOx]

Not relevant anymore. cssnext is now postcss-cssnext. cssnext is deprecated. See postcss-cssnext migration guide http://cssnext.io/postcss/