Add diamond kata exercise

diamond.md

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+## Diamond kata
+
+The diamond kata takes as its input a letter, and outputs it in a diamond 
+shape. Given a letter, it prints a diamond starting with 'A', with the 
+supplied letter at the widest point.
+
+## Requirements
+
+* The first row contains one 'A'.
+* The last row contains one 'A'.
+* All rows, except the first and last, have exactly two identical letters. 
+* The diamond is horizontally symmetric.
+* The diamond is vertically symmetric.
+* The diamond has a square shape (width equals height).
+* The letters form a diamond shape.
+* The top half has the letters in ascending order.
+* The bottom half has the letters in descending order. 
+* The four corners (containing the spaces) are triangles.
+
+## Examples
+
+Diamond for letter 'A':
+
+```plain
+A
+```
+
+Diamond for letter 'C':
+
+```plain
+  A
+ B B
+C   C
+ B B
+  A
+```
+
+Diamond for letter 'E':
+
+```plain
+    A       
+   B B   
+  C   C 
+ D     D
+E       E
+ D     D   
+  C   C
+   B B
+    A
+``` 

diamond.yml

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+---
+blurb: "Given a letter, print a diamond starting with 'A' with the supplied letter at the widest point."
+source: "Seb Rose"
+source_url: "http://claysnow.co.uk/recycling-tests-in-tdd/"