add chord references

src/testdata/minor_third.txt

@@ -0,0 +1,18 @@
+# minor third
+# +2 letters (the third letter) with exactly 1 half step between
+
+# c,e- c+,e d,f
+c  c+ d
+e- e  f
+
+# d+,f+ e,g f,g+
+d+ e  f
+f+ g  g+
+
+# f+,a g,a+ g+,b
+f+ g  g+
+a  a+ b
+
+# a,c a+,c+ b,d
+a  a+ b
+c  c+ d