Config file default uses open command on linux

Hi!

Just installed `lazycli` on Ubuntu 18.04 and tried it with `ls -l`. Typing `$` does open the config file with `xdg-open`, but the default config file commands to open files is `open`, which on linux means `openvt`. It's easy to change to `xdg-open` manually, but it was a bit confusing first.

Could it be possible to exchange the open command depending on the system it is installed on?