Improve handling of response to "do you want to publish to Grails Central"

At the moment, if the user says that they want to import the source code of a plugin into source control, they are then asked whether they want to add it to Grails Central Plugin Repository. The code handling the response to this should accept:

• No value, i.e. use default value of 'yes'
• 'y', 'Y', 'yes' (any case) for 'yes'
• 'n', 'N', 'no' (any case) for 'no'