Provide built in mechanism for custom sorting

[bdw429s]

99% of the time I'm fine with the default sorting of completion candidates. I've ran into a few scenarios where I would like to sort them differently (such as a list of items sorted by most recently accessed). I see there is a workaround provided in this issue to subclass the Candidate class and override the compareTo() method. I'm interacting with JLine from a JVM language (CFML) that adds enough boilerplate and runtime overhead to extend Java classes on the fly that this is not a very convenient option for me.

I would like to suggest the Candidate class introduce a new numeric sort member that defaults to 0 when not set. That will allow the default behavior to persist, but when creating my Candidate instances, I can pass a sort integer of my own creation to force an arbitrary sort order while still relying on the base compareTo() method.

[bdw429s]

I have taken a crack at my suggestion in this pull and it seems to work quite nicely in my local testing

#678

[bdw429s]

@gnodet @mattirn Any input on this?

[bdw429s]

@gnodet @mattirn Any input on this?