Default AssetSync.config.enabled in production/staging env

[davidjrice]

From a previous issue...

So I think possibly the best way is to have an Array of environments. (Defaulted to production and staging)

AssetSync.configure do |config|
  config.environments = [:production, :staging]
end

This could then be modified by the initializer.
Enabling via YAML and ENV variable might require accepting non-symbol environment names in a comma separated string.

[dgilperez]

+1

[dgilperez]

So this is my first attempt to achieve this feature.

Please share your thoughts about it, I can elaborate it forward if required.