Add micro-blog exercise

This is an exercise requiring students to truncate unicode strings.
Solves #1507

exercises/micro-blog/canonical-data.json

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+{
+  "exercise": "micro-blog",
+  "version": "1.0.0",
+  "comments": [
+    "This exercise is only applicable to languages that use UTF-8, UTF-16",
+    "or other variable width Unicode compatible encoding as their internal",
+    "string representation.",
+    "",
+    "This exercise is probably too easy in languages that use Unicode aware",
+    "string slicing.",
+    "",
+    "When adding additional tests to the problem specification, prefer tests",
+    "that pass and fail the same for UTF-8 and UTF-16.",
+    "",
+    "Avoid adding tests that involve characters (graphemes) that are made up",
+    "of multiple characters, or introduce them as a more advanced step.",
+    "",
+    "Consider adding a track specific hint.md about if your language uses",
+    "UTF-8, UTF-16 or other for its internal string representation. hkjhkjh "
+  ],
+  "cases": [
+    {
+      "description": "Truncate a micro blog post",
+      "cases": [
+        {
+          "description": "English language short",
+          "property": "truncate",
+          "input": {
+            "phrase": "Hi"
+          },
+          "expected": "Hi"
+        },
+        {
+          "description": "English language long",
+          "property": "truncate",
+          "input": {
+            "phrase": "Hello there"
+          },
+          "expected": "Hello"
+        },
+        {
+          "description": "English and emoji short",
+          "property": "truncate",
+          "input": {
+            "phrase": "Fly 🛫"
+          },
+          "expected": "Fly 🛫"
+        },
+        {
+          "description": "Emoji short",
+          "property": "truncate",
+          "input": {
+            "phrase": "💇"
+          },
+          "expected": "💇"
+        },
+        {
+          "description": "Emoji long",
+          "property": "truncate",
+          "input": {
+            "phrase": "❄🌡🤧🤒🏥🕰😀"
+          },
+          "expected": "❄🌡🤧🤒🏥"
+        }
+      ]
+    }
+  ]
+}

exercises/micro-blog/description.md

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+You have identified a gap in the social media market for very very short
+posts. Now that Twitter allows 280 character posts, people wanting quick
+social media updates aren't being served. You decide to create your own
+social media network.
+
+To make your product noteworthy, you make it extreme and only allow posts
+of 5 or less characters. Any posts of more than 5 characters should be
+truncated to 5.
+
+To allow your users to express themselves fully, you allow Emoji and
+other Unicode.
+
+The task is to truncate input strings to 5 characters.
+
+## Text Encodings
+
+Text stored digitally has to be converted to a series of bytes.
+There are 3 ways to map characters to bytes in common use.
+* **ASCII** can encode English language characters. All
+characters are precisely 1 byte long.
+* **UTF-8** is a Unicode text encoding. Characters take between 1
+and 4 bytes.
+* **UTF-16** is a Unicode text encoding. Characters are either 2 or
+4 bytes long.
+
+UTF-8 and UTF-16 are both Unicode encodings which means they're capable of
+representing a massive range of characters including:
+* English language text
+* Foreign language text
+* Historic text
+* Emoji
+
+UTF-8 and UTF-16 are both variable length encodings, which means that
+different characters take up different amounts of space.
+
+Consider the letter 'a' and the emoji '😛'. In UTF-16 the letter takes
+2 bytes but the emoji takes 4 bytes.
+
+The trick to this exercise is to use APIs designed around Unicode
+characters (codepoints) instead of Unicode codeunits.

exercises/micro-blog/metadata.yml

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+---
+title: "Micro Blog"
+blurb: "Given an input string, truncate it to 5 characters."