Add option to show electron window

[tylerjpeterson]

When testing in browser, I find it helpful at times to be able to view the window. This didn't appear possible when using electron as the browser. electron-stream already provides a show option, so I simply exposed the option via the bin script and updated the README.md.

I assume this qualifies as a "trivial" update, so I am forgoing the process outlined in the contributing guidelines.

[fregante]

Indeed, especially because Firefox already shows the window and I’d prefer the opposite. Perhaps the opposite of a --headless option? --headful?

[juliangruber]

I wouldn't mess with the term "headless", that gets confusing. Well if Firefox always shows we can consider "show: true" support to be done already :D