[ADDED] Add description support for completions

[cycoe]

Add description support for completions User can display descriptions when listing alternatives, by settting description field to `Just "some description"' in Completion data. If any alternative in current page is set with description, then single column mode is enabled automatically, like this:

0:
first         this is the first item
second        this is the second item
third         this is the third item
forth         this is the forth item
fifth         this is the fifth item

[cycoe]

@judah plz review this PR

[athas]

Is there still interest in this PR?