'Support both csso v1 and v2' - Improved code in handling csso optimization function (justDoIt vs. minify)

[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.

Tested with:

• require-css 0.1.8 as a base.
• 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 all possible values for optimizeCss

Compatibility Note: I realize, this change may be overthinking/overcomplex, but so far the fact is - there are end consumers of require-css who will upgrade to new require-css version, but remain old csso version --globalor as in my enterprise case (locally in project).
They will have errors like I mentioned in this PR and in #203 and #219.

A bit of automation :) in this boring life

1. In scope of this PR, I added compatibility-test.sh.
2. To use this script, you should change optimizeCss in example/build.jsfile from "none"to

• optimizeCss: "standard.keepLines"
• optimizeCss: "standard.keepWhitespace"
• optimizeCss: "standard.keepComments"
• optimizeCss: "standard.keepComments.keepLines"
• optimizeCss: "standard.keepLines.keepWhitespace" - default by r.js

3. At every execution time, script file generates compatibility.log. By searching value Compressed CSS output in that log file, u should see always all good, no Syntax Errors. Active for Feb-19-2017.

cc/ @guybedford @prantlf