Implement new two-fer exercise

exercises/two-fer/README.md

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+# Two Fer
+
+`Two-fer` or `2-fer` is short for two for one. One for you and one for me.
+
+```text
+"One for X, one for me."
+```
+
+When X is a name or "you".
+
+If the given name is "Alice", the result should be "One for Alice, one for me."
+If no name is given, the result should be "One for you, one for me."
+
+
+* * * *
+
+For installation and learning resources, refer to the
+[exercism Racket page](http://exercism.io/languages/racket).
+
+You can run the provided tests through DrRacket, or via the command line.
+
+To run the test through DrRacket, simply open the test file and click the 'Run' button in the upper right.
+
+To run the test from the command line, run the test from the exercise directory with the following command:
+
+```
+raco test two-fer-test.rkt
+```
+
+which will display the following:
+
+```
+raco test: (submod "two-fer-test.rkt" test)
+2 success(es) 0 failure(s) 0 error(s) 2 test(s) run
+0
+2 tests passed
+```
+
+## Source
+
+[https://en.wikipedia.org/wiki/Two-fer](https://en.wikipedia.org/wiki/Two-fer)
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/two-fer/example.rkt

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+#lang racket
+
+(provide hello)
+
+(define (two-fer [name "you"])
+  (string-append "One for " name ", one for me."))

exercises/two-fer/two-fer-test.rkt

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+#lang racket
+
+(require "two-fer.rkt")
+
+(module+ test
+  (require rackunit rackunit/text-ui)
+
+  (define suite
+    (test-suite
+     "two fer tests"
+
+     (test-equal? "no name given"
+                  (two-fer)
+                  "One for you, one for me.")
+     (test-equal? "a name given"
+                  (two-fer "Alice")
+                  "One for Alice, one for me!")
+     (test-equal? "another name given"
+                  (two-fer "Bob")
+                  "One for Bob, one for me!")))
+
+  (run-tests suite))

exercises/two-fer/two-fer.rkt

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+#lang racket
+
+(provide two-fer)