v0.6.0 beta release of Open PowerShell

With 274 non-merge commits since v0.5.0, here are the major changes:

- Targets .NET Core 1.0 release
- PowerShellGet enabled
- [system.manage<tab>] completion issues fixed
- AssemblyLoadContext intercepts dependencies correctly
- Type catalog issues fixed
- Invoke-Item enabled for Linux and OS X
- Windows ConsoleHost reverted to native interfaces
- Portable ConsoleHost redirection issues fixed
- Bugs with pseudo (and no) TTYs fixed
- Source Depot synced to baseline changeset 717473
- SecureString stub replaced with .NET Core package
