Add commands to open in other window or frame

Hi,

Thanks for writing this package, it's really useful. Though the one thing that's keeping me from using flx more is the fact that it can't open files in another window. This patch remedies that. The `fiplr-find-*-other-frame` commands were added for consistency.

I thought that a macro *could* be used here to reduce the lines of code, but I am uncertain if that would really help maintainability. If you disagree, I can easily change this code to use a macro.

Please let me know what you think.