if `patterns` is a string, use minimatch directly

This is a potential solution to https://github.com/sindresorhus/multimatch/issues/7. I still think the decision to not be inclusive by default makes sense when the glob patterns are arrays. However, I can also see why users might be confused when a string is passed.

With this solution, when an array of patterns is used nothing changes from current behavior, but when a string is used, we default to minimatch conventions which should be more consistent with user expectations all around. 

Also, this may not be immediately obvious, but `union` is used to uniquify the results, for consistency with current behavior.