Add `exports.extra` option for loading extra modules

With `exports.extra` option, you can load extra modules.

* `exports.extra` must be an array
Merge extra support. close #3
Release 0.2.0
Pass the original target to extra

Pass the original target to `exports.init` or `module.exports`
when automatic loading extra.

Add test cases.

Merge remote-tracking branch 'upstream/master'

Conflicts:
	lib/loading.js
	test/fixtures/controllers/extraWithExtra/index.js
	test/loading.test.js