Comment Outlining

Added OutliningTagger type that creates collapsible regions for blocks of single line comments and blocks of documentation comments. It works well as a complement to the [F# outlining extension](https://visualstudiogallery.msdn.microsoft.com/bec977b8-c9d9-4926-999e-e50c4498df8a).

![fsharp_comment_outlining](https://cloud.githubusercontent.com/assets/680051/5152580/219fb6d2-71c7-11e4-80e3-854e05b97ef3.png)

@vasily-kirichenko  sorry for how sloppy the last PR was
the only time `getDoccomCollapsed` is called is in

    let private buildDoccomRegions ls = (doccomBlocks ls |> parseRegionsOf) getDoccomCollapsed getDoccomTooltip

so it's only invoked in this match statement where it replaces `getCollapsed`

    match ls with
    | [] -> None
    | _  -> let fstline = ls.[0].LineNumber
            let lstline = ls.[ls.Length - 1].LineNumber
            if fstline = lstline then None else 
            let collapsed = getCollapsed ls
            let contents = getTooltip ls
            Some <| Region(fstline, lstline, collapsed, contents)

so if `lines` contains zero or one element it returns `None` and `getDoccomCollapsed` isn't invoked