I agree, but didn't want to worry about changing CzmlDataSource as part of this pull request.  Also, I wanted to factor out knowledge about each data source completely so that you don't suck in data sources you don't care about.  My plan is to create some sort of `DataSourceRegistry` which applications create/populate (and you can opt-in to use the defualt), but that's outside the code of this request.  If you feel strongly about it, I can modify CzmlDataSource.load as a stop-gap, but I'd rather look at all of this in another pull.