Comment Outlining 

I've added outlining for blocks of documentation comments and blocks of single line comments, it works well as a complement to the F# outlining extension.

![fsharp_comment_outlining](https://cloud.githubusercontent.com/assets/680051/5152580/219fb6d2-71c7-11e4-80e3-854e05b97ef3.png)

@vasily-kirichenko  is this around the amount you wanted for an early pull request?
`Seq.fold`?