Add commands to open in other window or frame

Changes the `fiplr-find-file-in-directory' and
`find-directory-in-directory' to accept an optional argument containing
the function to use to open the file. If no such function is specified
fall back to the `find-file' function, so as not to break any other
functions that might use either directly.
Merge pull request #34 from ryuslash/extra-functions

Add commands to open in other window or frame