[Extensions] Bind package.config's activation onLanguage to configuration

- VSCode Version: 1.18.0 dcee22 - OS Version: macOS 10.13.1 Steps to Reproduce: 1. Have an extension that has `"activationEvents": [ "onLanguage:markdown" ]` but allow users to [override the languages it works for via configuration](https://github.com/TravisTheTechie/vscode-write-good/blob/master/package.json#L22), e.g. adding text 2. The user now must load a markdown file before it will activate for text files It would be awesome if I could bind activation to whatever languages the user has configured instead of having to bind it to [every event](https://code.visualstudio.com/docs/extensionAPI/activation-events#_activationevents). Is this possible in some way that I can't find? Reproduces without extensions: No, it's extension related