Add simple snippets support

Author: Kyle-Ye

Snippets/Parsing/Explanation.md

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+Examples on how to use Swift Markdown Framework to parse the Markdown String into Swift object

Snippets/Parsing/ParseFile.swift

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+
+import Markdown
+import Foundation
+
+guard let url = Bundle.main.url(forResource: "Test", withExtension: "md") else {
+    fatalError("Unable to locate Test.md resource file")
+}
+guard let source = try? String(contentsOf: url) else {
+    fatalError("Unable to get the contents of the Test.md resource file")
+}
+let everythingDocument = Document(parsing: source)
+let document = Document(parsing: source)
+print(document.debugDescription(options: [.printEverything]))

Snippets/Parsing/ParseString.swift

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+
+import Markdown
+
+let source = "Hello world"
+let document = Document(parsing: source)
+print(document.debugDescription(options: [.printEverything]))

Snippets/Parsing/Resources/Test.md

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+1. Hello
+2. World

Snippets/Visiting/Explanation.md

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+Examples on how to use Swift Markdown Framework to visit the Markdown object

Snippets/Visiting/MarkupCounter.swift

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+//! Define a MarkupCounter struct to count the Text and UnorderedList count of the Markup Object 
+import Markdown
+struct MarkupCounter: MarkupWalker {
+    private var textCount = 0
+    private var unorderedListCount = 0
+    private var unorderedListItemsCount = 0
+
+    // MARK: Hide
+    var result: String {
+        """
+        The Markup has:
+        \(textCount) Text elements
+        \(unorderedListCount) UnorderedList elements with a total of \(unorderedListItemsCount) items
+        """
+    }
+    // MARK: Show
+
+    mutating func visitText(_ text: Text) -> () {
+        print("Visiting Text Element: \(text.string)\n")
+        textCount += 1
+    }
+    mutating func visitUnorderedList(_ unorderedList: UnorderedList) -> () {
+        print("Visiting UnorderedList Element: \(unorderedList.format())\n")
+        unorderedListCount += 1
+        unorderedListItemsCount += unorderedList.childCount
+    }
+}
+
+let source = """
+Hello
+
+- Item 1
+- Item 2
+
+world
+
+- Item 3
+"""
+let document = Document(parsing: source)
+
+var dumper = MarkupCounter()
+dumper.visit(document)
+print(dumper.result)