Implements neovim/neovim@fbf2c41 APIs

[zchee]

Implements neovim/neovim@fbf2c41 APIs.

diff:

> nvim_buf_del_keymap(buffer Buffer, mode String, lhs String) void { name(nvim_buf_del_keymap) }
> nvim_buf_set_keymap(buffer Buffer, mode String, lhs String, rhs String, opts Dictionary) void { name(nvim_buf_set_keymap) }
> nvim_del_keymap(mode String, lhs String) void { name(nvim_del_keymap) }
> nvim_set_keymap(mode String, lhs String, rhs String, opts Dictionary) void { name(nvim_set_keymap) }

Close: #53