config: add filter method.

lib/config.js

@@ -145,6 +145,31 @@ class Config {
     this.prefix = this.getPrefix();
   }
 
+  /**
+   * Create a child config. Filter by plugin name.
+   * @param {String} name
+   * @returns {Config}
+   */
+
+  filter(name) {
+    assert(typeof name === 'string');
+
+    const child = new Config(this.module);
+
+    child.prefix = this.prefix;
+    child.suffix = this.suffix;
+    child.fallback = this.fallback;
+    child.argv = this.argv;
+    child.pass = this.pass;
+
+    _filter(name, this.env, child.env);
+    _filter(name, this.args, child.args);
+    _filter(name, this.query, child.query);
+    _filter(name, this.hash, child.hash);
+
+    return child;
+  }
+
   /**
    * Set default option.
    * @param {String} key
@@ -1106,6 +1131,15 @@ function isUpperKey(key) {
   return !/[a-z]/.test(key);
 }
 
+function _filter(name, a, b) {
+  for (const key of Object.keys(a)) {
+    if (key.length > name.length && key.indexOf(name) === 0) {
+      const sub = key.substring(name.length);
+      b[sub] = a[key];
+    }
+  }
+}
+
 function fromFloat(num, exp) {
   assert(typeof num === 'number' && isFinite(num));
   assert(Number.isSafeInteger(exp));