@mention with space results in unexpected karma

Often times users greet/communicate with others following the pattern: "@mention_name -- blah blah blah"

This results in @mention_name's karma being reduced. Karma should only process karma for the previous mention without whitespace after it.

This does not occur without the @mention. i.e. "foo -- bar" does not decrease the string "foo"'s karma, but "@foo -- bar" would decrease the user foo's karma.