Add commands for checking for upgrade and upgrading src CLI in place

Created by: ryan-blunden

As a starting point, I like @slimsag 's suggestion from https://github.com/sourcegraph/src-cli/issues/18#issuecomment-425635219:

src update check -> checks for updates, doesn't apply them.
src update -> checks for updates, self-applies them. Will prompt for root/admin privileges.

I also liked @felixfbecker's suggestion of having a prompt to let the user know there is an upgrade available (screenshot taken from https://www.npmjs.com/package/update-notifier), but add logic so this is only displayed once per day, not on every invocation.