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.
Merge branch 'master' into 41-present_search_list_option
Merge pull request #43 from danchoi/41-present_search_list_option

allow searching with list-return through copen