Preprocessors strips docname from error reporting

[bsmith-n4]

Using a preprocessor in in the following way will strip the document name from error reporting:

Extensions.register {
  preprocessor {
    process {|document, reader|
      Reader.new reader.readlines.map {|l|
          # do something
       }
    }
  }
}

This occurs at the following two files:

• https://github.com/bsmith-n4/jekyll_aspec/blob/master/lib/extensions/autoxrefs.rb#L131
• https://github.com/bsmith-n4/jekyll_aspec/blob/master/lib/extensions/req_refs.rb#L81

Investigate if formal use will fix this or different handling of invoking the preprocessor.