Merge the hashes instead of explicit setting. Also switch the yml config
file to :foo: instead of foo: so the keys will be parsed as symbols.
This matches the @options keys.
See test/config.yml for an example file.
`sidekiq -C /path/to/config.yml` will use any settings set in the file.
Flags passed to sidekiq will override any set in the config file. So, if
config.yml looked like:
---
concurrency: 50
`sidekiq -C /path/to/that/config.yml -c 75` will set the concurrency to
75.