Opening PDF files not in default viewer

[dsanson]

As discussed in issue #71, it might be nice to allow the user to specify their preferred viewer for a given filetype. This could be via an option passed to PandocRegisterExecutor, or it could be via some global setting. It could involve passing options to the relevant system commands (e.g., the -a option the open command on OSX), or it could involve allowing the user to specify a custom launch command.

I am inclined to think this is more trouble than it is worth, but I am putting here so the suggestion doesn't get lost, and so users who would value this feature can chime in.