v6.0.0-alpha.11 release of PowerShell

With 108 non-merge commits since v6.0.0-alpha.10,
here are the major changes:

- Add '-Title' to 'Get-Credential' and unify the prompt experience
- Update dependency list for PowerShell Core on Linux and OS X
- Fix 'powershell -Command -' to not hang and to not ignore the last command
- Fix binary operator tab completion
- Enable 'ConvertTo-Html' in PowerShell Core
- Remove most Maximum* capacity variables
- Fix 'Get-ChildItem -Hidden' to work on system hidden files on Windows
- Fix 'JsonConfigFileAccessor' to handle corrupted 'PowerShellProperties.json'
and defer creating the user setting directory until a write request comes
- Fix variable assignment to not overwrite readonly variables
- Fix 'Get-WinEvent -FilterHashtable' to work with named fields in UserData of event logs
- Fix 'Get-Help -Online' in PowerShell Core on Windows
- Spelling/grammar fixes
