Reactive config

[jgraef]

We need config management with the following features:

• parse
• write
• react to file modifications
• fine-grained reactivity. We want to send signals to sub-systems when some of their configuration values change.
• transactional, fine-grained modifications (i.e. through UI components).
• good diagnostics (filename, line, column)
• generate documented default/example config (optional)
• TOML
• YAML. then we can use this for serde-yml parse with file location information #15