Remove DEBUG_FD

[TooTallNate]

Now simply uses process.stderr. Breaking API change, for the v3 branch.

Previously used internal and undocumented Node.js APIs to support this underly used API.

Fixes #280.
Closes #386.

[coveralls]

Coverage increased (+11.0%) to 74.419% when pulling 67bf20a on remove/debug_fd into a741173 on v3.

[develar]

And how now we can change output without editing client code? What WebStorm or electron-webpack should do?

[tanyaYourchuck]

@thebigredgeek @TooTallNate DEBUG_FD was removed entirely?
why not leave it whitelisted to 1 or 2 as was decided as resolution of #410?

My project had also used DEBUG_FD to make sure debug goes to stdout in all submodules as @wallali mentioned: Modules I depend on who also internally use debug whose code I do not control keep outputting to stderr and I cannot redirect them to stdout.

I do not want to redirect all stderr to stdout, just want all debug output to go to stdout. Is this doable without the DEBUG_FD flag?