allow searching with list-return through copen

Includes the following new commands:

  - SWSearchList which search the same way as SWSearch does
    but directly presents the user with a list of matches
    through copen (quickfix-list).
  - SWSL is an alias for SWSearchList
  - directly add SWS as alias for SWSearch to prevent
    ambiguous commands.

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should successfully implement #41.