Config values can be overwritten

[MisinformedDNA]

I noticed that user secrets were being overridden by local.settings.json. I did some research and here's how it happens:

The Azure Functions SDK sets the default configuration before this library can make it's changes. As such, all settings in the default configuration are added as environment variables.

After that is done, this library adds its own configuration values in the order of user secrets followed by environment variables. So if the environment variable is blank in the initial config, then the environments can just override what the user secrets just set.

For instance:

// local.settings.json
{
  "Secret": ""
}

```json
// secrets.json
{
  "Secret": "lsdkfdlds"
  "Password": "Passw0rd"
}

then environment values will end up with

Secret: ""
Password: "Passdw0rd"

This will be a breaking change so this will be done in a new version.