Let user choose line type in `plot` for `pdbart`

[AMBarbosa]

Hi, thanks for the great package. This is a simple-to-implement feature request, if there's no reason otherwise: would you add argument type='b' to the plot() call, and replace its 3 instances with type=type in the function body? This way the user could choose to use e.g. type='l' instead. Cheers!