Less strict matching of YARD tags.

This patch makes it possible for this plugin to match text such as
`#    @return` as YARD tags. Previously it would only match if there was
exactly 1 space between the comment character and the @ sign. This would
prevent the plugin from properly highlighting the following code:

    # @!attribute [r] example
    #  @return SomeConstant

Signed-off-by: Yorick Peterse <yorickpeterse@olery.com>