Allow arrays as providers

[weierophinney]

Per the modular skeleton discussion, we need to likely allow providing arrays to the ConfigAggregator constructor. This could be done in one of two ways:

• an ArrayProvider, which would accept an array to the constructor, and then simply return it on invocation.
• an actual array

The former would work with the current architecture, and only require an additional class.

The latter approach would likely require changing loadConfigFromProviders() to work differently, as it presently assumes a provider is a callable.