XCVersion: Add an option to reject beta versions

[TPXP]

Hello!

I'm running Fastlane in a CI environment with multiple XCode versions installed (Github actions' Mac OS).

The default XCode version was fine until now, but we're migrating to XCode 12 and so should our CI! So I added a call to xcversion to select the right XCode version.

  xcversion(version: "~> 12.0.0")

Which is okay, except it selects XCode 12's beta version!

[09:21:33]: -----------------------
[09:21:33]: --- Step: xcversion ---
[09:21:33]: -----------------------
[09:22:41]: Setting Xcode version to /Applications/Xcode_12_beta.app for all build steps

If possible, I'd prefer to have an beta option to prevent xcversion from picking this version. Something like this would be great:

xcversion(version: "~> 12.0.0", stable_only: true)

Thanks again for making and maintaining fastlane, it's my best ally for dealing with the iOS toolchain nightmare <3