Add common characters challenge

DIRECTORY.md

@@ -22,6 +22,7 @@
     * [Sort Squares Of An Array](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/sort_squares_of_an_array.rb)
     * Strings
       * [Anagram Checker](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/anagram_checker.rb)
+      * [Common Characters](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/common_characters.rb)
       * [Jewels And Stones](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/jewels_and_stones.rb)
       * [Remove Vowels](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/strings/remove_vowels.rb)
     * [Two Sum](https://github.com/TheAlgorithms/Ruby/blob/master/data_structures/arrays/two_sum.rb)

data_structures/arrays/strings/common_characters.rb

@@ -0,0 +1,19 @@
+# Challenge name: Find Common Characters
+#
+# Given an array A of strings made only from lowercase letters, return a list
+# of all characters that show up in all strings within the list
+# (including duplicates).  For example, if a character occurs 3 times in all
+# strings but not 4 times, you need to include that character three times in
+# the final answer.
+#
+# You may return the answer in any order.
+#
+# Example 1:
+#
+# Input: ["bella","label","roller"]
+# Output: ["e","l","l"]
+#
+# Example 2:
+#
+# Input: ["cool","lock","cook"]
+# Output: ["c","o"]