RFC: Updates to IO and default viewers

Two updates here.  The first is easy, config.jl now tests for "open", which will launch preview.app on a Mac, in preparation for imshow().

The second one might need a closer look.  Imagemagick's `identify` gives me class names with internal spaces:

```julia> imclass = strip(readline(stream))
"DirectClass Gray"
```

I changed the `classdict` entries, but I'm unsure if some of the entries will require additional spaces (`GrayAlpha` vs. `Gray Alpha`) because I don't have handy files that I can test.

(Can't wait to play with this some more, by the way... Thanks!)