resolve #763 -- choose multiselect for options with choices and nargs=+ or *

[jacadzaca]

fix #763

Example:

from gooey import Gooey, GooeyParser

@Gooey
def main():
    parser = GooeyParser()
    parser.add_argument(
        'some_argument',
        nargs='+',
        choices=['jaca', 'paca'],
    )
    argv = parser.parse_args()

    print(argv.some_argument)

if __name__ == '__main__':
    main()

TL;DR:

• [x] You're opening this PR against the current release branch
• [x] Works on both Python 2.7 & Python 3.x
• [x] Commits have been squashed and includes the relevant issue number
• [x] Pull request description contains link to relevant issue or detailed notes on changes
  • [x] This must include example code demonstrating your feature or the bug being fixed
• [x] All existing tests pass
• [x] Your bug fix / feature has associated test coverage