support multiple metrics per experiment

Allows experiments to support multiple "generic conversions" aka metrics. For example, [:add_to_cart, :purchase]. This way, triggering either finished(:add_to_cart) or finished(:purchase) will trigger a conversion on the experiment. 

My intended usage is to do finished(:add_to_cart => "add_to_cart") and finished(:purchase => "purchase") in order to track separate goals with metrics. I didn't try to automate it in order to maintain flexibility and backwards compatibility. Same with the name of the hash "metric." I would like to change it to "metrics" but that would break backwards compatibility. 

I don't fully understand the original intended use cases for metrics but from my point of view, it seems it should just automatically trigger conversions for a goal of the same name. In a sense, I see metrics as "universal goals" vs "experiment-specific goals." 

Does that make sense?