Add canonical-data.json for Nucleotide Count (#505)

Nucleotide count description:
https://github.com/exercism/x-common/blob/master/exercises/nucleotide-count/description.md

Closes https://github.com/exercism/todo/issues/116

Author: bunnymatic

exercises/nucleotide-count/canonical-data.json

@@ -0,0 +1,44 @@
+{
+  "nucleotide_counts": {
+    "description": "count all nucleotides in a strand",
+    "cases": [
+      {
+        "description": "empty strand",
+        "strand": "",
+        "expected": {
+          "A": 0,
+          "C": 0,
+          "G": 0,
+          "T": 0
+        }
+      },
+      {
+        "description": "strand with repeated nucleotide",
+        "strand": "GGGGGGG",
+        "expected": {
+          "A": 0,
+          "C": 0,
+          "G": 7,
+          "T": 0
+        }
+      },
+      {
+        "description": "strand with multiple nucleotides",
+        "strand": "AGCTTTTCATTCTGACTGCAACGGGCAATATGTCTCTGTGTGGATTAAAAAAAGAGTGTCTGATAGCAGC",
+        "expected": {
+          "A": 20,
+          "C": 12,
+          "G": 17,
+          "T": 21
+        }
+      },
+      {
+        "description": "strand with invalid nucleotides",
+        "strand": "AGXXACT",
+        "expected": {
+          "error": "Invalid nucleotide in strand"
+        }
+      }
+    ]
+  }
+}