JavaScript: Add an (empty) Customizations.qll module.

Somewhat analogous to the `Options.qll` module in C++; see module
comments for further explanation.

javascript/ql/src/Customizations.qll

@@ -0,0 +1,12 @@
+/**
+ * Contains customizations to the standard library.
+ *
+ * This module is imported by `javascript.qll`, so any customizations defined here automatically
+ * apply to all queries.
+ *
+ * Typical examples of customizations include adding new subclasses of abstract classes such as
+ * `FileSystemAccess`, or the `Source` and `Sink` classes associated with the security queries
+ * to model frameworks that are not covered by the standard library.
+ */
+
+import javascript

javascript/ql/src/javascript.qll

@@ -2,6 +2,8 @@
  * Provides classes for working with JavaScript programs, as well as JSON, YAML and HTML.
  */
 
+import Customizations
+
 import semmle.javascript.Aliases
 import semmle.javascript.AMD
 import semmle.javascript.AST