[folding] show folding lines

I would like to see more options with code folding and with bracket matching. It seems the user base is quite split on how they want the collapsed region to display, so I think this should be a user configuration setting along with the other features listed. ### Feature Requests 1. Collapse fold to a single line [Related#2 Alternative Designs#1](https://github.com/Microsoft/vscode/issues/2948#issuecomment-183540068) 1. Bracket anchors/leads/lines seen in `pictures #2 and #3` + [Related#2 Alternative Designs#2](https://github.com/Microsoft/vscode/issues/2948#issuecomment-183540068) 1. Peek at top bracket when hovering the bottom bracket `pictures #3` This should only happen when the top matching bracket isn't visible on the current viewport. 1. Highlight current region. In `pictures #1 and #2` it highlights the current block you're on 1. Collapse block from bottom bracket. In `pictures #1 and #3` you can collapse a block region from the lower bracket which can be really nice, it can be a quick way to see what the bracket belongs to w/o feature `request #3` ### Picture examples 1. Single line collapse  1. Anchor / leads / lines  1. Peek at top bracket  ### Related 1. Collapse to same line #3352 1. Icons for folding #2948 #### Labels - editor - editor-folding - feature-request