Plugin for test pages

After the refactoring this is the first pull request that allows using plugins, so far only to modify the content of a test page.

This is achieved by exposing some middlewares and adding the modules

- plugins where you can require a plugin from disk or npm
- testPage where you can add element into a test page

For testing purposes, this pull request introduces also an attester.dispose to cleanly dispose all objects created.

Conflicts:
	lib/attester/cli.js
	lib/test-type/aria-templates/at-test-type.js
	lib/test-type/base-test-type.js
	lib/test-type/mocha/mocha-test-type.js