Support initial menu delay

[ask4fukumi]

Problem

whichkey.delay is a nice setting to delay the appearance of the menu, but it not just only delay the initial level of showing the menu, but all of them. Having to wait xxx miliseconds to wait for the next level of keys to appear or to go back a level is really annoying.
Example here.

Solution

I suggest adding a setting or changing it whole delay behavior to just delay at the initial menu showing.

[stevenguh]

Seems reasonable.

Just drafting what I am thinking: To implement this without adding another whichkey.initialDelay, maybe something like whichkey.delayModel to control the delay mode.