RFC: Reconsider behavior when `-C` is not passed

[aturon]

Currently if you don't provide -C, Viceroy will simply not load any configuration.

Normally -C is automatically provided by the Fastly CLI, but when invoking Viceroy directly it probably makes sense to default to fastly.toml if no manifest is provided. If fastly.toml is not found, then we can proceed without configuration.

Thoughts?

[cratelyn]

This sounds like a wise choice to me!