Allow to use xxdi, lightweight alternative to vim's "xxd -i"

Allow to use xxdi, lightweight alternative to vim's "xxd -i"

xxdi [0] is a lightweight script that can replace "xxd -i", which is now
needed to build geeqie. This is helpful on systems not running vim.
This checks for its availability on the system and falls back to
existing vim mode if not installed.

[0] https://github.com/gregkh/xxdi