Add initial_mode to settings to control pickers initial mode (insert / normal)

[dfeeley]

Hi,
Like in #74 I prefer to start the picker in normal mode as I'm usually grabbing the most recent couple of yanks so j/k is quicker than filtering. While that issue mentions the solution :Telescope neoclip initial_mode=normal I think it would be neater to add initial_mode as a setting.
PR to follow.
thanks, love the plugin btw.