Don't share global options object for calls to PostCSS

[mdjzon]

The fields "to" and "from" is set in a global options object for each processed css file. After postcss is finished the chained function that writes the result to disk might get totally different values for "to" and "from" thus writing the css to the wrong file.

I clone the options object using Object.assign to avoid this.

[coveralls]

Coverage remained the same at 95.333% when pulling 484d47d on mdjzon:master into 0ac612c on postcss:master.

[RyanZim]

I've personally been suspicious that code might be problematic; I meant to refactor this sometime, but I never got around to it.

I'm not sure this approach is right; I'm thinking we should rename the global config to cliConfig or something like that. We'd then have have rc() actually return a config object without mutating the global cliConfig. If you feel up to trying this, you're welcome to do so. If not, I'll try to do it myself as time allows.