Add support for beta and nightly channels

[wknapik]

curl ...|sh
curl ...|CHANNEL=release sh
curl ...|CHANNEL=beta sh
curl ...|CHANNEL=nightly sh

[fmarier]

Adding MVP label just so that we either remove the CHANNEL variable throughout the script in V1, or (ideally) make it work as described in issue description.

[wknapik]

In #31 the CHANNEL variable is already removed.

I think we should publish this with support for release only and possibly add support for other channels later.

Let's get it out ASAP and gather feedback.

[fmarier]

Ok, then it's not an MVP issue.

[wknapik]

Instead of a copy of the install.sh widget per channel at brave.com/linux, we could just add radio buttons to select the channel, that would add the appropriate variable to the one-liner.

We could also reuse this approach for setting Brave as the default browser - have a checkbox for it.

And for uninstallation - another radio button.

All the buttons could be hidden under an Advanced button that would toggle their display.