Add cross-platform support to show-in-file-manager.

 The "Show in file manager" menu option should now open the preferred file manager on Windows and GNU/Linux (and probably *BSD) as well as Mac OS X.

Unfortunately, I overlooked this pull request when I wrote this patch, so some duplication of effort took place:
https://github.com/atom/tree-view/pull/141

(my apologies to @bradgearon for that)