Create SimpleMode.js

Have the function SimpleMode(arr) take the array of numbers stored in arr and return the number that appears most frequently (the mode). For example: if arr contains [10, 4, 5, 2, 4] the output should be 4. If there is more than one mode return the one that appeared in the array first (ie. [5, 10, 10, 6, 5] should return 5 because it appeared first). If there is no mode return -1. The array will not be empty.