Implement config diff tool

Implement a tool that can build/rebuild a `config.toml` file from a description of changes.

For example:
Defined change:
```
pex=false
```

Result:
A `config.toml` with the default settings but pex set to false.

Poor man's choice is sed, but we have a whole library to manage the config.toml so I'm pretty sure we can come up with a more versatile (and stable) tool.

Reason:
Currently, each experiment's node configuration is managed separately and understanding differences requires `diff` (and possibly slightly changed config files). It would be better if we could just define the differences to the baseline default config for any experiment so it's easier to handle the configuration.