Add new arguments to get_planit()

[Robinlovelace]

As documented here #69 (comment)

In the function definition you have mentioned 'krad' for the radius of a circle search with 'pcode' as the centre but you can also define the centre directly using 'lat and 'lng'

There is also a 'bounds' parameter where you can supply a boundary polygon but not sure if that is of use