Add example using Pywebview

I have got this working, but it's decidedly rough:

Pywin32 is compiled with an older version of the MSVCR (2010), which the user has to download separately and install.
The resulting application is really slow, practically unusable even on a new laptop. The culprit appears to be something in comtypes which generates code containing True = 1, which is a syntax error in Python 3. I think that the error loading this generated code causes it to keep trying to generate it again. When I comment that line out, it runs at a reasonable speed.
Building on Windows probably won't work, because it relies on the bash script fetching and arranging things. It should be easy enough to translate to Python if anyone wants to do that, though.

takluyver / pynsist