Should be able to find all installers for all versions of unity in one call

[jwittner]

Right now Find-UnitySetupInstaller requires you to specify a version. It'd be great if I could specify -Latest or neither to find all versions.

• [ ] Should patch and beta versions be included by default if no args are passed?
• [ ] Should we add flags to control the versions you'll search for, use param sets to avoid Version being specified.
• [ ] Should we add a Find-UnitySetupVersion which finds available versions which could be piped to 'Find-UnitySetupInstaller'?

[jwittner]

Folding into 4.0 for now as it sits alongside #87