Write a function called digit_counter

 Write a function called digit_counter that takes the name of a text file as input and returns the number of digits (i.e., any of the characters, 0-to-9) that the file contains. If there is a problem opening the file, the function returns -1. WARNING: if you use the ‘w’ flag with fopen, as opposed to ‘r’, you will overwrite the file. The grader uses your own m files to check your function, so be careful!