Support merging of multiple ConfigurationSource in ConfigurationProvider

[dvdgsng]

Being able to merge multiple ConfigurationSources into on ConfigurationProvider would be helpful when dealing with configurations from files and environment variables.

I'm thinking something like this:

ConfigurationProvider configurationProvider = new ConfigurationProviderBuilder()
    .withConfigurationSource(new FilesConfigurationSource(configFilesProvider))
    .withConfigurationSource(new EnvironmentVariablesConfigurationSource() )
    .build();

or

ConfigurationProvider configurationProvider = new ConfigurationProviderBuilder()
    .withConfigurationSources(new FilesConfigurationSource(configFilesProvider), new EnvironmentVariablesConfigurationSource())
    .build();

... where the EnvironmentVariablesConfigurationSource has the higher priority due to being applied 2nd.