Please provide an option to lock the screen only a while after blanking

## I'm submitting a…

[ ] Bug

[x] Feature Request

[ ] Other (Please describe in detail)

## Current Behavior

i3lock unconditionally immediately locks the screen when started

## Wanted Behavior

An option which allows a (usually short) delay during which the screen is already blanked/saved, but not yet locked, e.g. any mouse movement or keypress just exits i3lock again.

This is common on mobile phone (a common setting seems 5 minutes) and also present in XScreenSaver.

Common use case: You're sitting in front of the screen thinking or explaining something on the screen or talking with someone in the same room, and you just poke the mouse once the screen goes blank instead having to enter your super long password.

## Fictional Example

```
i3lock --lock-delay 5
```

(Lock the screen only 5 seconds after blanking it.)

## Environment
Output of `i3lock --version`:

i3lock: version 2.14.1 © 2010 Michael Stapelberg