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-function.js

@@ -0,0 +1,2 @@
+function foo() { return (1 + 2) + 3; }
+foo();