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.
Fix typo
Add parent group to fiplr customization group

This way the fiplr settings can be properly discovered from Emacs'
customize interface, without having to know exactly what the name of the
module is. Convenience was picked because it is the first of the
package's keywords and the only existing customization group among those
keywords.
Merge pull request #34 from ryuslash/extra-functions

Add commands to open in other window or frame
Version bump to 0.2.7
Merge branch 'extra-functions'
Merge pull request #35 from ryuslash/small-fixes

Small fixes
Ensure clearing the cache always yields a new list

By using the `list' function we ensure we don't reuse the same value
like when using a quoted list literal.
Merge pull request #37 from ryuslash/cache

Ensure clearing the cache always yields a new list