Auto obtain release notes from version history

[bbondy]

Obtain release notes from:
https://github.com/brave/browser-laptop/releases/tag/v0.7.8

That way we don't need to pass them via command line and you can just grab the markdown there.
On the browser side we'll make the release notes viewer show markdown.

[bbondy]

Actually maybe it should grab it from the local browser-laptop repo's CHANGELOG.md file (by parsing it) and then the script can upload the release to github with release notes too like:
https://github.com/atom/electron/blob/master/script/cibuild does.