code: function resolvePluginFiles (collection, relPath) {
      return Immutable.fromJS(collection.reduce(function (all, item) {
          var full = path.join(relPath, item);
          if (fs.existsSync(full)) {
              all[full] = fs.readFileSync(full, "utf8");
          }
          return all;
      }, {}));
  }
docs: Read & store a file from a plugin
  @param {Array|Immutable.List} collection
  @param {String} relPath
  @returns {any}