v0.5.0 beta release of Open PowerShell

With 256 non-merge commits since v0.4.0, here are the major changes:

- Paths given to cmdlets are now slash-agnostic (both / and \ work as directory separator)
- Lack of cmdlet support for paths with literal \ is a known issue
- .NET Core packages downgraded to build rc2-24027 (Nano's build)
- XDG Base Directory Specification is now respected and used by default
- Linux and OS X profile path is now `~/.config/powershell/profile.ps1`
- Linux and OS X history save path is now `~/.local/share/powershell/PSReadLine/ConsoleHost_history.txt`
- Linux and OS X user module path is now `~/.local/share/powershell/Modules`
- The `~/.powershell` folder is deprecated and should be deleted
- Scripts can be called within PowerShell without the `.ps1` extension
- `Trace-Command` and associated source cmdlets are now available
- `Ctrl-C` now breaks running cmdlets correctly
- Source Depot changesets up to 715912 have been merged
- `Set-PSBreakPoint` debugging works on Linux, but not on Windows
- MSI and APPX packages for Windows are now available
- Microsoft.PowerShell.LocalAccounts is available on Windows
- Microsoft.PowerShell.Archive is available on Windows
- Linux xUnit tests are running again
- Many more Pester tests are running
