Add Website option to ignore paths by name regex

[atomicbird]

This replaces #121, which I'll remove. I'm making this update because I mistakenly sent #121 from the master branch instead of a feature branch. Sorry for any confusion.

This adds an optional ignoredPaths array to Website which may contain a list of regular expressions of file or folder names that should be ignored during publishing. Each file or folder name is compared to entries in the array using range(of: name, options: .regularExpression), so wildcards may be used. A structure that adopts Website may optionally declare something like this to affect the output:

    var ignoredPaths: [String]? { ["templates", "skip-this-file*"] }

My motivation for this is that I have a folder of template documents used on my site, which should not be copied to the site output.