Allow command chaining for :ClearSwapList and :SwapIdea

Especially for :ClearSwapList, one may want to put this into
    let b:undo_ftplugin .= '|ClearSwapList'

To enable appending other such commands, the command must support chaining, and because it doesn't take arguments, this is trivially done through -bar.
Merge pull request #18 from inkarkat/allow-command-chaining

Allow command chaining for :ClearSwapList and :SwapIdea