Error when trying to load multiple config files

[lucascaton]

Hi there! jekyll server supports multiple config files:

$ bundle exec jekyll server -h | grep config
            --config CONFIG_FILE[,CONFIG_FILE2,...]  Custom configuration file

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It'd be nice to have it in rack-jekyll as well:

Rack::Jekyll.new config: '_config.yml,spec/_config.yml'

Fatal: The configuration file '_config.yml,spec/_config.yml' could not be found.

Thank you.