enable <!-- --> comments for Markdown

[kevinushey]

(NOTE: not quite ready for merge)

This PR:

1. Rewrites the doCommentUncomment() method to be line-by-line, rather than targeted on a whole selection (using regexes). This makes it easier to control how commenting works for 'double-sided' comments and also makes it a bit easier to handle what happens when considering commenting a line containing only whitespace.

2. Using the above, enables <!-- --> comments for Markdown modes.

As is tradition, a GIF showing this in action in an R Markdown document. Note that lines containing only whitespace within a selection are commented when within R mode, but not when within Markdown mode. (This seems like the most natural behaviour to me)

[kevinushey]

One main difference between the new and old scheme is that the new scheme respects indentation within blocks, e.g.

    foo <- function(x) {
        print(1)
    }

is indented as

    # foo <- function(x) {
    #    print(1)
    # }

instead of

#     foo <- function(x) {
#         print(1)
#     }

I believe this is an improvement (and this is also the behaviour when we comment a single line); thoughts?

[jjallaire]

This looks great! (I've often been frustrated by the fact that comment/uncomment doesn't work in markdown). Also agree 100% with the new indenting behavior. Let me know once you think this is ready for merge and I'll take it.

[kevinushey]

Should be good to go now!

[jjallaire]

Could you add a NEWS.md item for this?

[kevinushey]

Done!