Allow adapters to load multiple sections.

The section specification parameter in the constructor to be mixed:

string - Names a single section to be loaded (currently supported)

array - Array of strings, which are the names of multiple sections to be loaded

null - Indicates that all sections should be loaded

When multiple sections are loaded, first level properties would correspond with section names (e.g., $config->sectionName). When a single section is loaded, then the first level properties will be the first level of the data space of that section ($config->key1).