Make multiple colorschemes available.

[otakutyrant]

Hello, I used to pick random colorsheme from multiple themes that are installed in my neovim config. However, due to there is only a colorscheme of onedark.nvim and the style is decided independently, I cannot do that to onedark.nvim. So I convert all styles to colorschemes to offer users more flexible choices. Do you have interest in accepting the PR?

This is the first time I used Lua and modified a neovim plugin, so I am not sure whether it is backward-compatible.