Make statusline customizable from Lua

Maybe have some sort of an API like this:
```lua
require'uivonim'.setup_statusline {
    {}; -- Left components in here
    {}; -- Middle components in here
    {}; -- Right components in here
}
```

One question: how to go about this once ext_statusline is implemented in neovim?

cc: @tjdevries, let me know if you would prefer a different API style