Add possibility to customize path to binary using alias.

If you don't want to install command-line utils globally, you have to specify it's paths.

    public $commands = [
        'styl' => ['css', '@app/node_modules/bin/stylus < {from} > {to}'],
    ];

close #5130

framework/web/AssetConverter.php

@@ -25,6 +25,14 @@ class AssetConverter extends Component implements AssetConverterInterface
      * @var array the commands that are used to perform the asset conversion.
      * The keys are the asset file extension names, and the values are the corresponding
      * target script types (either "css" or "js") and the commands used for the conversion.
+     *
+     * You may also use a path alias to specify the location of the command:
+     *
+     * ```php
+     * [
+     *     'styl' => ['css', '@app/node_modules/bin/stylus < {from} > {to}'],
+     * ]
+     * ```
      */
     public $commands = [
         'less' => ['css', 'lessc {from} {to} --no-color --source-map'],
@@ -63,7 +71,7 @@ class AssetConverter extends Component implements AssetConverterInterface
 
     /**
      * Runs a command to convert asset files.
-     * @param string $command the command to run
+     * @param string $command the command to run. If prefixed with an `@` it will be treated as a path alias.
      * @param string $basePath asset base path and command working directory
      * @param string $asset the name of the asset file
      * @param string $result the name of the file to be generated by the converter command
@@ -73,6 +81,8 @@ class AssetConverter extends Component implements AssetConverterInterface
      */
     protected function runCommand($command, $basePath, $asset, $result)
     {
+        $command = Yii::getAlias($command);
+        
         $command = strtr($command, [
             '{from}' => escapeshellarg("$basePath/$asset"),
             '{to}' => escapeshellarg("$basePath/$result"),