code: function addFile(filename) {
          if (ignoredPaths.contains(filename)) {
              return;
          }
          filename = fs.realpathSync(filename);
          if (added[filename]) {
              return;
          }
          files.push(filename);
          added[filename] = true;
      }
docs: Executes the linter on a file defined by the `filename`. Skips
  unsupported file extensions and any files that are already linted.
  @param {string} filename The file to be processed
  @returns {void}