Make the config a component of the app

[jpmckinney]

So that other components can get the config options directly from it.

For example, the poller and scheduler currently store the config, so that they can call get_spider_queues with it.

First mentioned in #186

[jpmckinney]

Temporarily added Root._config.

See the TODO and links to https://github.com/scrapy/scrapyd/issues/526 in the code.