Add support to css plugins

`data-uri` is now implemented as plugin

css.js

@@ -7,6 +7,30 @@ exports.CSSStyleSheet = CSSStyleSheet
 
 var fs = require("fs")
 , path = require("path")
+, read = (sheet, url, enc = "utf8") => fs.readFileSync(path.resolve(sheet.min.root || "", sheet.baseURI, url).split(/[+#]/)[0], enc)
+, plugins = exports.plugins = {
+	"data-uri": function(sheet, v) {
+		var { DOMParser } = require("./dom.js")
+		return v.replace(urlRe, function(_, q1, q2, url) {
+			if (q1) return _
+			var frag = url.split("#").pop()
+			, ext = url.split(/[?#]/)[0].split(".").pop()
+			, enc = ext === "svg" ? "utf8" : "base64"
+			url = read(sheet, url, enc)
+			if (ext === "svg") {
+				url = new DOMParser().parseFromString(url, "application/xml")
+				if (frag && (frag = url.getElementById(frag))) {
+					frag.removeAttribute("id")
+					url.documentElement.childNodes = frag.tagName === "g" ? frag.childNodes : [ frag ]
+				}
+				url = url.toString(true).replace(/#/g, "%23")
+				enc = ""
+				ext += "+xml"
+			}
+			return "url('data:image/" + ext + ";" + enc + "," + url + "')"
+		})
+	}
+}
 , urlRe = /(["']).*?\1|url\((['"]?)(?!\/|data:|https?:)(.*?)\2\)/g
 , clearFn = (_, q, str, c) =>
 	q ? (q = str.indexOf("'") == -1 ? "'" : "\"", q + str.replace(q === "'" ? /\\(")/g : /\\(')/g, "$1")) + q :
@@ -19,7 +43,6 @@ var fs = require("fs")
 , clear = s => s
 	.replace(/("|')((?:\\\1|[^\1])*?)\1|\s*(\/)\*(?:[^*]|\*(?!\/))*\*\/\s*|(?:[^"'\/]|\/(?!\*))+/g, clearFn)
 	.replace(/(["']).*?\1|url\(("|')([^'"()\s]+)\2\)/g, (m,q1,q2,u) => q1 ? m : "url(" + u + ")")
-, read = (sheet, url, enc = "utf8") => fs.readFileSync(path.resolve(sheet.min.root || "", sheet.baseURI, url).split(/[+#]/)[0], enc)
 , toRgb = {
 	rgb(r, g, b) {
 		var f = n => ((n | 0) + 256).toString(16).slice(1)
@@ -63,7 +86,7 @@ var fs = require("fs")
 			, lastIdx = {}
 			for (; (m = re.exec(val)); ) {
 				if (m[4]) {
-					if (min && len) style[k = style[len - 1]] = clear(transformValue(m[4].trim(), style[k]))
+					if (min && len && plugins[m[4] = m[4].trim()]) style[k = style[len - 1]] = clear(plugins[m[4]](sheet, style[k]))
 					continue
 				} else {
 					k = m[1]
@@ -78,30 +101,6 @@ var fs = require("fs")
 			if (!style[prop]) style[style.length++] = prop
 			style[prop] = style[prop === "cssFloat" ? "float" : prop.replace(/[A-Z]/g, "-$&").toLowerCase()] = clear(val)
 		}
-		function transformValue(cmd, v) {
-			var { DOMParser } = require("./dom.js")
-			if (cmd === "data-uri") {
-				return v.replace(urlRe, function(_, q1, q2, url) {
-					if (q1) return _
-					var frag = url.split("#").pop()
-					, ext = url.split(/[?#]/)[0].split(".").pop()
-					, enc = ext === "svg" ? "utf8" : "base64"
-					url = read(sheet, url, enc)
-					if (ext === "svg") {
-						url = new DOMParser().parseFromString(url, "application/xml")
-						if (frag && (frag = url.getElementById(frag))) {
-							frag.removeAttribute("id")
-							url.documentElement.childNodes = frag.tagName === "g" ? frag.childNodes : [ frag ]
-						}
-						url = url.toString(true).replace(/#/g, "%23")
-						enc = ""
-						ext += "+xml"
-					}
-					return "url('data:image/" + ext + ";" + enc + "," + url + "')"
-				})
-			}
-			return v
-		}
 	}
 }
 , ruleTypes = {