Allow custom settings files for host

With the DI/IConfiguration work we're doing, this will likely be possible from the host side, but is this something we can expose to customers authoring in VS?

See https://stackoverflow.com/questions/50003241/add-json-config-file-to-azure-function-for-local-development for request.

Start by reviewing the DI/IConfiguration work referenced in the issue and the linked Stack Overflow request about adding a JSON configuration file for local development. Determine how the host-side capability could be exposed to customers authoring Azure Functions in Visual Studio; done means a concrete, supported custom-settings-file experience is defined and implemented.

Written by the indexing model from the issue text.