Graceful optional `enter` and `exit`

[ryuheechul]

enter and exit` being optional makes it sound like I can omit the entire key but that was not true and only reading the error message didn't give any clue on that.

So only after reading the source code of this plugin, I was able to figure the conclusion of below.

1. works

local layer = KeyLayer({
   enter = {},
   layer = {...},
   exit = {}
})

2. doesn't work

local layer = KeyLayer({
   layer = {...},
})

I think 2. should work as well.

[ryuheechul]

btw, it's a brilliant plugin!