Support `about:blank` as `startup-url`

[martin-braun]

Setting about:blank as startup-url will result in searching for "about:blank" on the search engine at launch instead of simply displaying an empty page. I really don't like landing pages.

[ylluminate]

In addition to this "about:blank" (which is great and would help a lot), it would also be fantastic if it focused on the address bar input for immediate input of a url. This would help the startup ergonomics of browsh somewhat significantly. Another thing that might help is that on the blank page, instead of being entirely blank, print out the keybindings (https://www.brow.sh/docs/keybindings/) as a quick reference. This would not be visually overwhelming and would give a nice starting point.

I might also add that with these additions, it would be nicer as having this all as the default web page loading up instead of the browsh homepage... I think, again, this would help the ergonomics / user experience.