Support providing extra flags to `start()` and `restart()`

[disberd]

Initial attempt at fixing #7 by providing a way to give extra flags for the kaleido process. Fixes #7

This would allow passing explicitly the URL of the desired plotly version using kwargs.

I based the flags parsing code on the original python code inside the Kaleido library here:

https://github.com/plotly/Kaleido/blob/3b590b563385567f257db8ff27adae1adf77821f/repos/kaleido/py/kaleido/scopes/base.py#L116-L141

I also added a small sleep inside the restart() function due to the issue presented in #8