abolish the post-install script and template/plugin copying (#519, #612)

JSDoc has a few requirements that are somewhat unusual for a Node.js app:

1. We need `require('jsdoc/foo')` to work from any module.
2. We need `require('jsdoc/foo')` to work from external code, such as templates and plugins.

Prior to this commit, JSDoc did two separate things to meet these requirements:

1. Use an npm post-install script to create a symlink from `lib/jsdoc` to `node_modules/jsdoc`.
2. When a user runs JSDoc, copy templates and plugins into the JSDoc directory.

These fixes worked, sort of. But they also caused numerous issues with file permissions, especially on Windows.

We now use the Requizzle module, which hacks the Node.js module system to support JSDoc's use cases. There's no longer a post-install script, and there's no need for a symlink in `node_modules`.