Ensure that validatePaths can process arrays of globs

I discovered that it's not whilst you can specify a ** path as a String in yuidoc.json, it's not possible to use an array of ** strings, or to use the ** pattern in the CLI.

Looking into it, the String works because minimatch is only used to expand the ** pattern if the value is not an array.
Ensuring that paths is always an array and then performing the minimatch on all members of the array has the desired effect.