Add change-first? function

[liquidz]

I added change-first? function. This function set whether on-chnage function will be execute at startup or not.

At startup, watchtower execute on-change function for all files one by one, and if there are many files, this behavior will cause slow startup.

[zcaudate]

I have the same problem too, because I don't want to be executing functions on files that I have previously. I've renamed the option to be 'notify-on-start?' on my fork in addition to a couple of other changes. Please have a look at my fork to see what you think