Read config options from file on init

I've added a `read_config` method to `Options` which defaults to reading configuration options from the `.s3_sync` file in the MM project root. You can also pass it any IO object and it will read options from there (dependency injection ftw!)

The constructor for `Options` now calls `read_config` so the config file will always be read, no matter where it is instantiated.

I have also removed the old code in `commands.rb` that read the config file as this new code supersedes it.

This fixes #22.