Add support for buffer keymaps

[tapayne88]

Hi 👋

Thanks for the great project!

In the process of swapping over to this to wrap my keybinding function I found that it currently doesn't support keybindings bound to a buffer.

To implement I've pretty much just ripped off the neovim internal implementation. https://github.com/neovim/neovim/blob/574a5822023939d534d922eaa345bb7e0633d2b8/runtime/lua/vim/keymap.lua#L82-L88

What do you think?