feat(fzf-lua): option to override fzf-lua picker opts

[barrett-ruth]

This change addresses issue #83. Now one can setup neoclip and override the prompt variable, for example, with the following configuration:

require('neoclip').setup {
    keys = {
        fzf = {
            opts = {
                prompt = 'clip> '
            }
        }
    }
}

[barrett-ruth]

Actually, I agree. Just made ability to pass a prompt (for both telescope and fzf-lua).