- Introduced an internal boolean property 'overridable'
- Set this property to true while registering configuration
- Support inheriting this property from parent configuration node level

- Use '[...identifiers]` as key

- Register the settings.override setting
- Dynamically update the schema to support intellisense
- Change the language key format to langid or langids separated by comma

- Contirbute parsing of config as an option to config watcher
- Introduce Config model to hold configurations , overrides and necessary data for services
- Refactor config services to use config models
- Add an api in config services to get configuration for a language

see https://basarat.gitbooks.io/typescript/content/docs/tips/nominalTyping.html and https://github.com/Microsoft/TypeScript/blob/7b48a182c05ea4dea81bab73ecbbe9e013a79e99/src/compiler/types.ts#L693-L698