Add Metalsmith.static method for pass-through file copy

[webketje]

Adding a metalsmith.static('media/**') method with a signature similar to metalsmith.ignore would allow metalsmith to use native NodeJs fs.copyFile to do a pass-through file copy, which would avoid overhead from reading files into memory that are then not even processed.

TBD whether files matched with metalsmith.static should figure as object keys in the Files object.
If they do, they must not have valid files as values. Still this might break backwards-compatibility with some plugins.

'copiedFile.zip': { }
//or
'copiedFile.zip': null

So a better option would perhaps be to have metalsmith.static() (without arguments) return the list of matches (so plugins know exactly which files will be/ have been copied over)

In any case a test is needed with a large file to see the effect on perf & interoperability

Note: aligning metalsmith.ignores() to return the ignored matches (instead of the existing & equally accessible ignores property)