Replace paths doesn't handle substrings correctly, and only replaces first instance of a url

[MalucoMarinero]

Hey there, I find staticify real handy in a prebuild and deploy scenario, there's just a few issues with replacePaths that mean it doesn't currently work completely for my use case:

1. Substrings are often incorrectly replaced

@font-face {
  font-family: 'Portrait Web';
  src: url('/assets/font-portrait/Portrait-Regular-Web.eot');
  src: url('/assets/font-portrait/Portrait-Regular-Web.eot?#iefix') format('embedded-opentype'),
       url('/assets/font-portrait/Portrait-Regular-Web.woff2') format('woff2'),
       url('/assets/font-portrait/Portrait-Regular-Web.woff') format('woff');
  font-weight:  400;
  font-style:   normal;
  font-stretch: normal;
}

Consider CSS that looks like this, if Object.keys returns the woff path before the woff2 path, the hash for the .woff path will actually end up on the .woff2, meaning a broken link in production.

Should be easily fixed with a length sort of Object.keys(versions), so longer strings get handled before shorter strings.

2. String.replace only replaces the first instance when used with strings.
   Usually not an issue in my builds, but if a file is referenced more than once replacePaths won't currently pick it up. Better to turn it into a new RegExp(url, "g"), the global flag will ensure it gets everything.

I think this code should fit the bill but I didn't want to create a pull request and be presumptive, let me know if you want me to create one.

    const replacePaths = fileContents => {
        return Object.keys(versions).sort(function(a, b) { return b.length - a.length })).reduce((f, url) => {
            return f.replace(new RegExp(url, "g"), getVersionedPath(url));
        }, fileContents);
    };

Cheers!

[XhmikosR]

You are welcome to make a PR with the fix and tests 👍

[MalucoMarinero]

Done, I'll leave it with you. Passes xo and mocha test.