Allow repl_open_cmd to be custom function for dynamic views

[jakubkaczor]

Preferred behavior

Setting repl_open_cmd to custom function makes iron call this function to determine the view. This lets setting the view dynamically. For example, one could define a function with a conditional statement, so that when the window is in landscape proportion, the window is split vertically, but otherwise horizontally. For example

repl_open_cmd = function()
    if vim.o.columns < 158 then
        return iron.view.bottom(15)
    end
    return iron.view.right(79)
end,