Don't mutate options

[vweevers]

PR #5 was meant to fix options being mutated between files, but only works when no options are passed. This PR creates a copy of the options for every entry.

Additionally, I did a small cleanup of the tests for readability. For example, in this

new Buffer(fs.readFileSync(
    path.resolve(__dirname, "fixtures/index.css"),
    {encoding: "utf8"}
))

you can:

• Shorten {encoding: "utf8"} to "utf8"
• Or even leave it out, you want a buffer anyway
• And because this snippet is used multiple times, I made a helper function:

function read(file, encoding) {
  return fs.readFileSync(path.resolve(__dirname, file), encoding)
}

[MoOx]

can you use "object-assign" ponyfill instead of xtend?
Anyway, you should read this lionelB/cssnext-brunch#10 (comment)

[vweevers]

OK, replaced xtend.

Thanks for the link, though I'm just trying out postcss, so I wanted to start with a nice "preset" of plugins (and not spend too much time).