Parse double quotes in urls in css as well

Fixes #13
Note that the regex was also updated to only have single quotes once between the '[]'.

src/index.js

@@ -144,7 +144,7 @@ class DissectHtml {
     const self = this
     // to get -> property: url(filepath)
 
-    const processed = text.replace(/url\(['|']?([^)]+?)['|']?\)/ig, function (_u, url) {
+    const processed = text.replace(/url\(["'|]?([^"'|)]+?)["'|]?\)/ig, function (_u, url) {
       // to get -> filepath from url(filepath), url('filepath') and url('filepath')
       return `url(${self._changeRelUrl(url, path.dirname(cssBasePath))})`
     })