wordcount challenge

The classic word-count algorithm: given an array of strings, return a Map<String, Integer> with a key for each different string, with the value the number of times that string appears in the array.

wordCount(["a", "b", "a", "c", "b"]) → {"b": 2, "c": 1, "a": 2}
wordCount(["c", "b", "a"]) → {"b": 1, "c": 1, "a": 1}
wordCount(["c", "c", "c", "c"]) → {"c": 4}