allow disabling prompt for cmdheight=0

[jemag]

Is there a way to disable the prompt?

Currently with cmdheight=0, this will will make using sneak a bit painful since you will get an enter confirmation every time you trigger it :

Using an empty prompt:

vim.g["sneak#prompt"] = ''

will still trigger the prompt, but just remove the > character.

[justinmk]

PR welcome to change the behavior for vim.g["sneak#prompt"] = 'NONE'