Make PowerShell ExecutionContext available to C#
Update version number to 13
Add color support for an error status message
Add ValidateAndAcceptLine function

To avoid cluttering the screen and history with command lines
that don't work because of stupid errors like "command not found",
you can bind enter to ValidateAndAcceptLine.
Make Enter default to ValidateAndAcceptLine
Update changes.txt
Add -ValidationHandler for ValidateAndAcceptLine

To support custom validation, for e.g. to allow someone
to reject command lines where arguments are not valid,
you can now specify a custom validation handler script block.
Merge pull request #156 from dsolodow/master

PowerShell script updates