Add options to set the width and height of floating window

[xtrats]

Is your feature request related to a problem? Please describe. No

Describe the solution you'd like Add options to set the width and height of floating window (kind = "floating"), such as

require('neogit').setup {
  kind = 'floating',
  windows  = {
    width = 0.7,   -- percentage
    height = 0.8 
}

Describe alternatives you've considered No for now