use argparse, compare input/output mod dates, new option to force update

`argparse` recommended by PEP389. Comparing input/output dates saves from unnecessary conversions and is cheaper than caching. The `--force` argument is useful when the user changes in `tex` the options with which `dot2tex` will be called, but not the `dot` source. It is available in [`\includedot`](https://github.com/kjellmf/dot2texisty/commit/2588e9d4318adab459941b9c2bcb885bebc021a2#diff-26e6a1db32a79dbc9c5b236fa2da211aR234).