config: Allow for "cloud friendly" configuration

[myme]

Experience when working with Azure Functions has shown that in some situations it's undesirable to implicitly load a configuration file if one is found.

The development workflow for Azure Functions is to launch and test them locally, but without much isolation or sandboxing. This means that users that have novem setup regularly could contaminate the test by having a ~/.config/novem/novem.conf accessible to the function.

For these cases, being able to easily set default configurations and completely ignore any possible configuration files would be useful. There are several options (and not necessarily mutually exclusive):

1. Support more environment variables: NOVEM_CONFIG_FILE, NOVEM_API_ROOT, etc.
2. Allow setting default_config from scripts: novem.default_config({ ... }).
3. ???

Environment variables are typically well supported in cloud runtime environments. So they should probably have a high precedence.

[myme]

I don't think I quite understand what I was asking for with this. The CLI has --config-path as shown in #170 and that makes it quite easy to skip loading the config file, it that's what you want.