Support Array for `patternDir`

[dmolsen]

sourceDir (or sourceDirs) should be able to be defined as an array as a package creator may place assets in different locations.

[EvanLovely]

Been wanting this for awhile; it'd be really helpful! Wouldn't we want an array of source/_patterns/ directories though? An array of sourceDirs could result in multiple _data/data.jsons; how would that get handled, merging?

[dmolsen]

You're correct. I've updated the title of the ticket to reflect that. Just got focused on the *Dir options that are available and patternDir isn't among them. Guess it needs to be :)