[feature request] Support keychains as per sxhkd

[draxil]

A major feature of sxhkd is keychains, which are not supported by swhkd.

I would propose matching this feature, indeed I'll probably explore adding it if nobody beats me to it.

This example from the sxhkd shows how it's configured there:

super + o ; {e,w,m}
	{gvim,firefox,thunderbird}

So here super+o followed by w will open firefox.