nvim: support neovim/neovim@907fc8a APIs (#144)

* nvim: support neovim/neovim@907fc8a APIs

> nvim_cmd(cmd Dictionary, opts Dictionary) String { name(nvim_cmd) }
> nvim_parse_cmd(str String, opts Dictionary) Dictionary { name(nvim_parse_cmd) }
> nvim_set_hl_ns(ns_id Integer) void { name(nvim_set_hl_ns) }
> nvim_set_hl_ns_fast(ns_id Integer) void { name(nvim_set_hl_ns_fast) }
> nvim_win_set_hl_ns(window Window, ns_id Integer) void { name(nvim_win_set_hl_ns) }

* nvim: go generate

* nvim: add Cmd and ParseCmd testcase

* nvim: add SetHighlightNamespace testcase

Author: zchee