Plugin configure command

This PR addressed http://teampulse.telerik.com/view#item/275633 and also I need it to implement this story http://teampulse.telerik.com/view#item/275633
Just to clarify, there are two options here:
1) Use only canExecute method and validate the parameters in it. This is the approach that should be used when you do not know the exact number of parameters or the validation is very special and cannot be inside command parameter.
2) Use only PluginConfigureCommandParameter as allowedParameters (remove canExecute method). This is useful when you know there's exact number of parameters. From the code above it looks like this command accepts only one parameter, so I think you should use this approach :)