Automatically set 2-space indenting when opening Dart files #32

Code doesn't support per-language settings yet, so it's likely users want this but can't achieve it. Add an opt-out setting.

Most of this is already done but it didn't seem to work correctly (see microsoft/vscode#10048), but since I've added a wrapper (util.getConfig<T>) it should be simply to put back (setting removal is here)