Implement a config system

- [x] ConfigSource
- [x] ConfigFileSource
- [x] ConfigDefaultSource
- [x] ConfigGlobalSource
- [ ] ConfigProjectSource
- [x] ConfigCmdArgsSource
- [ ] ConfigCommentSource
- [ ] ConfigHeadCommentSource
- [ ] vint.ast.parser.plugin Plugin
- [ ] vint.ast.parser.plugin.scope Scope
- [ ] ConfigScopeCommentSource
- [ ] Config
- [ ] Config.handleComment