feat: add mappings api

[max397574]

closes #14

examples

vim.keymap.set("i", "<cr>", "<Plug>(NeocompleteConfirm)")
vim.keymap.set("i", "<tab>", function()
    local mappings = require("neocomplete").mappings
    if mappings.is_open() then
        mappings.select_next(1)
    elseif luasnip.is_available() then
        luasnip.jump(1)
    end
end)

todo:

• see if fallback can be created only when mapping is executed to allow creating them on startup (potentially before user mapped them)