Support both csso v1 and v2

[alundiak]

csso releases review:

• since 1.4.0 we have minify() and earlier (eg. 1.3.12) it was ONLY justDoIt()
• in csso 1.8.0 csso.minify() and csso.justDoIt() both exists and they are functions.
• in csso 2.0.0 and 2.3.1 has no justDoIt() AT ALL.

I did deep research of how your suggested change can affect potential end customers/consumers of require-css.

And here is my code change suggestion:

try {
  if (typeof csso.minify === 'function') {
      var minifyResult = csso.minify(css);
      if (typeof minifyResult === 'string'){ // for csso < 2.0.0
        css = minifyResult; 
      } else if (typeof minifyResult === 'object'){ // for csso >= 2.0.0
        css = minifyResult.css; 
      } 
  } else { // justDoIt() was always. minify() appeared in csso 1.4.0.
    css = csso.justDoIt(css);
  }
}

I did this, because I know enterprise projects, which still use old version of require-css and csso, so my approach is universal. Tested with:

• require-css 0.1.8.
• csso 1.3.12, 1.4.0, 1.8.0, 2.0.0 and latest (checked 2017/02/18) 2.3.1.
• requirejs 2.1.10 and 2.3.2
• node 6.9.5 + npm 3.10.10
• node 7.5.2 + npm 4.1.2
• command r.js -o example/build.js with optimizeCss: "standard"

Note: I realize, this change may be overthinking/overcomplex, but so far the fact is - if we merge ur current code change, someone who will upgrade to new require-css version, but remain old csso version, they will have errors like I mentioned in this PR and in #203.

So @prantlf please consider my code above as a suggestion to apply to your PR. After 1 week silence, I will close this PR, and recreate new one with my suggested code.

cc/ @guybedford