feat(postcss-plugin.js): update for css-loader@4

css-loader@4 introduced a breaking change which meant the use of `~` needs to be relocated from
before the `!!` to after the loader use + `!`

BREAKING CHANGE: this change requires css-loader@4 to work, earlier versions of css-loader should
use previous version

re #46

.travis.yml

@@ -1,8 +1,7 @@
 language: node_js
 node_js:
-  - 6
-  - 8
   - 10
+  - 12
 before_install:
   - stty columns 120
 install:

README.md

@@ -19,11 +19,11 @@ This plugin requires:
  + webpack 3.x or higher 
  + postcss-loader 2.x or higher
  + node 6 or higher
- + css-loader 2.x or higher
+ + css-loader 3.x or higher
 
 ## Installation
 
-```
+```bash
 npm i --save-dev postcss-loader
 npm i --save-dev iconfont-webpack-plugin
 ```
@@ -84,7 +84,6 @@ const IconfontWebpackPlugin = require('iconfont-webpack-plugin');
                   resolve: loader.resolve,
                   fontNamePrefix: 'custom-',
                   enforcedSvgHeight: 3000,
-                  modules: false,
                 })
               ]
             }

appveyor.yml

@@ -3,7 +3,7 @@
 
 environment:
   matrix:
-    - nodejs_version: 6
+    - nodejs_version: 10
 
 version: "{build}"
 build: off

lib/postcss-plugin.js

@@ -158,7 +158,7 @@ function addFontDeclaration (fontName, postCssRoot, enforcedSvgHeight, svgPaths)
   const iconFontLoaderPath = path.join(iconFontPluginDirectory, 'lib/loader.js').replace(/\\/g, '/');
   const iconFontPlaceholderPath = path.join(iconFontPluginDirectory, 'placeholder.svg').replace(/\\/g, '/');
   // Use !! to tell webpack that we don't want any other loader to kick in
-  const url = '~!!' + iconFontLoaderPath + '?' + JSON.stringify(options) + '!' + iconFontPlaceholderPath;
+  const url = '!!' + iconFontLoaderPath + '?' + JSON.stringify(options) + '!~' + iconFontPlaceholderPath;
   postCssRoot.prepend(postcss.parse(
     '@font-face { ' +
     'font-family: ' + fontName + '; src:url(\'' + url + '\') format(\'woff\');' +