formatValues should handle a list of IDs

[zbraniecki]

At the moment, formatValues requires to pass an array of arrays of [id, attrs], but quite often the attrs are not needed and the additional array is not necessary.

it would be nice if instead of:

      let [title, message, okButton, cancelButton] = await document.l10n.formatValues([
        ["containers-remove-alert-title"],
        ["containers-remove-alert-msg"],
        ["containers-remove-ok-button"],
        ["containers-remove-cancel-button"]
      ]);

one could write:

      let [title, message, okButton, cancelButton] = await document.l10n.formatValues([
        "containers-remove-alert-title",
        "containers-remove-alert-msg",
        "containers-remove-ok-button",
        "containers-remove-cancel-button"
      ]);

but if we decide not to support it, it would at least be helpful to throw when attempting to pass a string. (but then, if we're already detecting that... how about make it work for our friend, the developer?)