Resolve #122 supporting Markdown Fenced Code Block

Syntax highlighting inside markdown code blocks:

```elixir
Graph.match(
  from person in "Person",
    where: person.age < 32 and person.title == "Developer",
    join: friend in "Person", on: "FRIEND",
    join: fof, on: "FRIEND",
    join: work, on: "WORKED_WITH",
    select: {person.name, friend.name, fof.name, work.name}
)
```

Inspired by https://github.com/mjbvz/vscode-fenced-code-block-grammar-injection-example

package.json

@@ -59,6 +59,14 @@
         "language": "HTML (Eex)",
         "scopeName": "text.html.elixir",
         "path": "./syntaxes/html (eex).json"
+      },
+      {
+        "scopeName": "markdown.elixir.codeblock",
+        "path": "./syntaxes/codeblock.json",
+        "injectTo": ["text.html.markdown"],
+        "embeddedLanguages": {
+          "meta.embedded.block.elixir": "elixir"
+        }
       }
     ],
     "snippets": [

syntaxes/codeblock.json

@@ -0,0 +1,28 @@
+{
+  "fileTypes": [],
+  "injectionSelector": "L:markup.fenced_code.block.markdown",
+  "patterns": [
+    {
+      "include": "#plantuml-code-block"
+    }
+  ],
+  "repository": {
+    "plantuml-code-block": {
+      "begin": "elixir(\\s+[^`~]*)?$",
+      "end": "(^|\\G)(?=\\s*[`~]{3,}\\s*$)",
+      "patterns": [
+        {
+          "begin": "(^|\\G)(\\s*)(.*)",
+          "while": "(^|\\G)(?!\\s*([`~]{3,})\\s*$)",
+          "contentName": "meta.embedded.block.elixir",
+          "patterns": [
+            {
+              "include": "source.elixir"
+            }
+          ]
+        }
+      ]
+    }
+  },
+  "scopeName": "markdown.elixir.codeblock"
+}