Add support for showing Flow messages to the User via an output channel

[orta]

Bah, looks like the a change online endings for flowMain as with #53 - so far this seems to be the only file that has this issue, is it worth having it switched over (like these diffs are doing?)

This PR adds the ability for users to see any problems, perviously to debug a "why isn't this working?" you would have to open the web inspector for VS Code.

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I initially tried building this via log4js' appender system, but I couldn't get it to ever echo anything out.

I'm not 100% on whether it's a good idea to add VS Code specific bits inside any lib/pkg/nuclide-* but it looks like the libraries are pretty mature on nuclide's sides, and so it feels like an ok idea.

[gabelevi]

Awesome! For some reason I had it in my head that writing to an output channel would cause that channel to pop up whenever stuff was written to it. But I checked out your branch and it seems to work great! Nice!

I'm not 100% on whether it's a good idea to add VS Code specific bits inside any lib/pkg/nuclide-* but it looks like the libraries are pretty mature on nuclide's sides, and so it feels like an ok idea.

At this point I think it's a pretty hard fork, so I'm ok with this.