Add support for grouped keys

[musjj]

If you have a lot of keys that you want to use for lazy loading, your config can become a bit hairy:

use {
  "FooBar/Baz",
  keys = {
    { "n", "a" },
    { "n", "b" },
    { "n", "c" },
    { "n", "d" },
    { "v", "e" },
    { "v", "f" },
    { "v", "g" },
    { "v", "h" },
  },
}

With this PR, you can now instead group your keys like this:

use {
  "FooBar/Baz",
  keys = {
    n = { "a", "b", "c", "d" },
    v = { "e", "f", "g", "h" },
  },
}

[musjj]

Anything blocking this from getting merged? To clarify, this PR is backwards-compatible with the original way of specifying keys.

[XXiaoA]

Hey @musjj! thanks for your great PR, which I really need. Therefore, when will we fix the docs?🤗