Support Beta Stream Releases

[leaanthony]

Currently wails update pulls the latest release regardless of what it is. We need a way to be more flexible with this, supporting the following modes:

• Update to latest stable
• Update to latest preview
• Update to specific version

[luizfonseca]

Hi @leaanthony,
I'd vote for something like rustup does + the concept of "channels":

1. wails update will fetch and update the current 'branch' for the user
2. wails update --all will fetch all "channels" (release, master, develop, preview etc)
3. wails update --target 0.12.3 for specific versions
4. wails use release if we want to test the release branch
5. wails use preview, wails use 0.12.3 etc

These are just suggestions, but i'd also recommend to maybe restrict to 3 of these channels to just start in a 'MVP' state.

Let me know if it makes sense!

[leaanthony]

That's fantastic feedback! I don't think we're going to need too many channels and maybe the 2 we currently have will be enough. It feels like rustup probably just uses git commands whereas go modules work a little differently. Let me know how you get on with the way it currently works and provide feedback in a couple of weeks? Thanks for your suggestions @luizfonseca!