Plugins - support that a plugin can trigger a diagnostic validation when its configuration changes

The diagnostics produced by the tslint language server extension depend on the tslint configuration file `tslint.json`. When the user edits this file and adds/removes a rule, then plugin needs to be able to trigger a revalidation of the diagnostics. Diagnostics have a `source` property which is set to the originating plugin, only these diagnostics need to be recomputed. 