Add missing keys on context of EachHelper

[giflw]

Add @key to Iterable and @index to Map as in handlebars.js, where iterables and maps use the same helper function execIteration (https://github.com/wycats/handlebars.js/blob/master/lib/handlebars/helpers/each.js) (2016-09-27).

line 22:

    function execIteration(field, index, last) {
      if (data) {
        data.key = field;
        data.index = index;
        data.first = index === 0;
        data.last = !!last;
      }

      ret = ret + fn(context[field], {
        data: data,
        blockParams: [context[field], field]
      });
    }

line 36:

 if (context && typeof context === 'object') {
      if (isArray(context)) {
        for (let j = context.length; i < j; i++) {
          if (i in context) {
            execIteration(i, i, i === context.length - 1);
          }
        }
      } else {
        let priorKey;

        for (let key in context) {
          if (context.hasOwnProperty(key)) {
            // 3 lines of comments....
            if (priorKey !== undefined) {
              execIteration(priorKey, i - 1);
            }
            priorKey = key;
            i++;
          }
        }
        if (priorKey !== undefined) {
          execIteration(priorKey, i - 1, true);
        }
      }
    }

[coveralls]

Coverage increased (+0.02%) to 86.226% when pulling 1380e37 on giflw:patch-3 into 1f6c48e on jknack:master.