`nanobot init` should create a `nanobot.toml` file

[jamesaoverton]

When we run nanobot init for the first time, it should create a new nanobot.toml file and populate it with some default values. It's probably best to define a default_config.toml somewhere in the source tree, and copy that. The default config should specify the Nanobot version number.

If we run nanobot init and the nanobot.toml file already exists, then we should exit with an error.

[jamesaoverton]

The nanobot.toml file should be stored in version control. It should usually contain just a small number of config fields for which we can't specify a default, and whatever overrides to the defaults that the user has specified.