Tidy-up of argument parsing & default handling

Updates:
* argparse parameters no longer have their own defaults
* everything optional goes in kwargs rather than being enumerated
* all parameters are now documented in the main docstring

New bits:
* bocho will now optionally delete the output file before re-drawing
* fixed a bug in the border drawing that meant it only drew the top + left
* desired DPI of PDF -> PNG page conversion is now an option