open: change core open method to keywords instead of positionals.

[StefanKarpinski]

Closes #3556.

[ViralBShah]

Much nicer API!

[StefanKarpinski]

Hmm. I'm not entirely sure. The thing that bothers me is the default of the read keyword being true while all the others default to false. It feels really inconsistent. But I can't think of a better way to do it.

[JeffBezanson]

Stefan and I discussed that the definition we'd need is

function open(file; write=false, read=!write)

which keyword args don't officially support but probably could.

[StefanKarpinski]

So we're not doing this? I think it would be better if we had the ability to define read=!write.