Add test for implicit paragraphs

Author: wooorm

test/fixtures/paragraph-implicit/index.html

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+<section>
+  <h1>Example of paragraphs</h1>
+  This is the <em>first</em> paragraph in this example.
+  <p>This is the second.</p>
+</section>

test/fixtures/paragraph-implicit/index.json

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+{
+  "fragment": true
+}

test/fixtures/paragraph-implicit/index.md

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+# Example of paragraphs
+
+This is the _first_ paragraph in this example.
+
+This is the second.