Task List comments

After seeing how desired this feature is at [user voice](http://vfpt.uservoice.com/forums/247560-general/suggestions/5773087-todo-comments) I decided to attempt to implement it.

This is what I have so far. I'd be willing to invest some more time into improving it by implementing:
- updating Task List when files are modified from outside of VS
- navigate from task list entry to location in code file
- other useful ideas

Those are just things to experiment with. I don't know how hard/possible it would be to implement them.

I don't have the means to test this on VS14, so I'd appreciate if you could do that. 2012 and 2013 work fine.

Thanks!

P.S. Just let me know if I should change anything

**EDIT**: I noticed AppVeyor failed to build this PR for the same reason it failed to build others 3 days ago (something about "NUnit test failed"). Is there something I can do to fix it or is it normal?
I think we can get rid of mutability here, like this:
```fsharp
member __.GetOptions() =
        dte.get_Properties("Environment", "TaskList")
        |> Seq.cast
        |> Seq.tryPick (fun (prop: Property) ->
            if prop.Name = "CommentTokens" then
                Some (prop.Value :?> obj[] |> Array.map (fun o -> o :?> string))
            else None)
        |> function
           | Some ct ->
                ct |> Array.choose (fun s -> 
                    match s.Split(':') with
                    | [| comment; priority |] -> Some { Comment = comment; Priority = Int32.Parse priority } 
                    | _ -> None)
           | None -> [| CommentOption.Default |]
```