Add new command "projectile-ff-find-other-file"

This command is used for switching between files with the same name but
with different extensions in the current project. It would be useful for
switching between header files and source files in C/C++, an possibly
test files with the same names.
Normally we follow the convention of adding a separate command for another window. See something like `projectile-find-implementation-or-test-other-window`.