feat: allow merging of keymap preset with custom keymap

[kedom1337]

This PR adds the ability to merge a custom keymap with one of the keymap presets.
This is achieved by

1. Extending the keymap configuration with a preset key which specifies which preset to use
2. Using vim.tbl_extend to then merge the preset with the custom keymap

Example:

keymap = {
  preset = 'default',
  ["<cr>"] = { "select_and_accept", "fallback" },
},

[Saghen]

Looks great thanks! Some conflicts on the readme since I just added an enter preset

[Saghen]

Thanks again!

[kedom1337]

I should thank you :). Thanks for the awesome plugin!