Feature: add encoding: utf-8 property to File

[webketje]

Before trying to parse front-matter metalsmith.readFile does a utf-8 check, however this information is lost in the build although it could be useful. Proposal would be to add a file.encoding property to files which pass the check

[webketje]

Another thought would be to add file.utf8 instead, because after all what we are most interested in is whether a file is textual content or binary, to determine whether we can do string manipulation. Mime types can be easily added with an npm pkg like https://www.npmjs.com/package/mime-types based on file extension, but the utf-8 check is more expensive because it has to walk through the file byte by byte