js: Take the script file as a command-line argument

Now that we have the beginnings of a parser, let's take the script to
run as a command-line argument and move all the test scripts into
/home/anon/js :^)

To run a script, simply use "js":

$ js my-script.js

To get an AST dump before execution, you can use "js -A"

Base/home/anon/js/simple-variables.js

@@ -0,0 +1,7 @@
+c = 1;
+function foo() {
+    var a = 5;
+    var b = 7;
+    return a + b + c;
+}
+foo();