Add special on-load command

[SeerLite]

Same as on-cd but executed everytime the load command is called.

My use-case is I want to show the number of hidden files in the prompt (actually in the status line but I found no statuslinefmt (#310? #112?) so I settled for the prompt). It works relatively fine with on-cd but it breaks if a file is created while lf is open or if the file is created via lf.

And thanks again for making the best file manager <3 Out of all software I use daily I'd say lf is the only one that gets the least in my way and does exactly what I expect it to.

[joelim-work]

I think the closest thing we have is on-select which will trigger if the currently selected file changes.

But if you just wanted to show the number of hidden files in the status bar, you can use %h in the rulerfmt option. Someone else requested the same thing in #1287 which is now supported.