Fixes 730 (picking up changes in _config.yml when run with --auto)

This commit enables jekyll to pick up changes in the _config.yml file when run with --auto (--watch in the current master).

It does so by detecting if the set of changed files includes the _config.yml file, and if so, it updates the current site options with the hash from the config file and then processes it.

I tested this by adding a destination option to the `_config.yml` and it worked. The way this is currently done might be improved, as I am using the same code to both initialize and update site options. Perhaps we can selectively update options based on whats passed from the config file but I only had a couple of hours today. 

This commit LACKS tests. I opened this pull request to get early feedback and get a sense of whether this is the correct approach to the issue. It obviously needs more work. If green lighted, I will add tests and update the docs.