Adding of a new exercise printalt.

The goal: printing the alphabet alternatively with the first and last letter first and so on. azbycx etc...

go/tests/prog/printalt_prog/main.go

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+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("azbycxdwevfugthsirjqkplomn")
+}

go/tests/prog/printalt_test/main.go

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+package main
+
+import (
+	"lib"
+)
+
+func main() {
+	lib.ChallengeMain("printalt")
+}

subjects/printalt/README.md

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+## printalt
+
+### Instructions
+
+Write a program that prints the Latin alphabet in lowercase alternatively with the first and last letters, then with the second and second to last letters, and so on until all the alphabet letters are displayed on a single line.
+
+A line is a sequence of characters preceding the [end of line](https://en.wikipedia.org/wiki/Newline) character (`'\n'`).
+
+Please note that `casting` is not allowed for this exercise!
+
+### Usage
+
+```console
+student@ubuntu:~/[[ROOT]]/printalt$ go build
+student@ubuntu:~/[[ROOT]]/printalt$ ./printalt
+azbycxdwevfugthsirjqkplomn
+student@ubuntu:~/[[ROOT]]/printalt$
+```