Add language-specific _language settings_

[lunelson]

Although vscode has (as of recently) language-specific editor settings, I would like to request language-specific language settings i.e. the ability to override definitions in a language's language-configuration.json file (found in vscode/extensions/[language]/).

For example let's say I would like to set the backtick as a "surrounding pair" character in pug, so I can select some text and mark it as an es6 string literal by hitting backtick (like I can do in JavaScript), and not have the backtick character replace the selection: ideally, as a setting, this would require only something like this in my "User Settings" file, to override pug's surroundingPairs definition:

  "[pug]": {
    "surroundingPairs": [
      ["{", "}"],
      ["[", "]"],
      ["(", ")"],
      ["`", "`"],
      ["'", "'"],
      ["\"", "\""]
    ],
  },

[alexdima]

I think this is best handled by tweaking the pug surrounding pairs. Surrounding pairs for a language should be based on the syntax of said language. So, if ` are used for strings in pug, then ` should be in the surrounding pairs.