Selectively cache config

[jesseleite]

It makes sense to cache marks, but I don't think it makes sense to cache the new menu config proposed by #46.

For example, if you add the following:

require('harpoon').setup {
  menu = {
    width = 50,
    height = 8,
    borderchars = {'!', '@', '$', '%', '^', '&', '*', '|'},
  }
}

Then remove that config, it'll keep reading that from harpoon.json. I feel like if you remove cosmetic menu config, it should revert back to uncached defaults.