Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/shell/solutions/great_day.md

@@ -0,0 +1,15 @@
+## Great Day
+
+### Objectives
+
+1. Write a script that will print "Today is a great day!" unless it's given a day name and then it should print "Today is <given day>"
+
+Note: no need to check whether the given argument is actually a valid day
+
+### Solution
+
+```
+#!/usr/bin/env bash
+
+echo "Today is ${1:-a great day!}"
+```