Extract blog post truncation logic and add tests (#640)

lib/core/__tests__/__fixtures__/blog-post-with-truncate.md

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+---
+title: Truncation Example
+---
+
+All this will be part of the blog post summary.
+
+Even this.
+
+<!--truncate-->
+
+But anything from here on down will not be.
+
+Not this.
+
+Or this.

lib/core/__tests__/__fixtures__/blog-post-without-truncate.md

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+---
+title: Non-truncation Example
+---
+
+All this will be part of the blog post summary.
+
+Even this.
+
+And anything from here on down will still be.
+
+And this.
+
+And this too.

lib/core/__tests__/__snapshots__/utils.test.js.snap

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+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`utils extractBlogPostBeforeTruncate 1`] = `
+"---
+title: Truncation Example
+---
+
+All this will be part of the blog post summary.
+
+Even this.
+
+"
+`;
+
+exports[`utils extractBlogPostBeforeTruncate 2`] = `
+"---
+title: Non-truncation Example
+---
+
+All this will be part of the blog post summary.
+
+Even this.
+
+And anything from here on down will still be.
+
+And this.
+
+And this too.
+"
+`;
+
+exports[`utils extractBlogPostSummary 1`] = `
+"---
+title: Truncation Example
+---
+
+All this will be part of the blog post summary.
+
+Even this.
+
+<!--truncate-->
+
+But anything from here on down will not be.
+
+Not this.
+
+Or this.
+"
+`;
+
+exports[`utils extractBlogPostSummary 2`] = `
+"---
+title: Non-truncation Example
+---
+
+All this will be part of the blog post summary.
+
+Even this.
+
+And anything from here on down will still be.
+
+And this.
+
+And this too.
+"
+`;

lib/core/__tests__/utils.test.js

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+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const path = require('path');
+const utils = require('../utils');
+const readFileSync = require('fs').readFileSync;
+
+const blogPostWithTruncateContents = readFileSync(
+  path.join(__dirname, '__fixtures__', 'blog-post-with-truncate.md'),
+  'utf-8'
+);
+
+const blogPostWithoutTruncateContents = readFileSync(
+  path.join(__dirname, '__fixtures__', 'blog-post-without-truncate.md'),
+  'utf-8'
+);
+
+describe('utils', () => {
+  test('blogPostHasTruncateMarker', () => {
+    expect(utils.blogPostHasTruncateMarker(blogPostWithTruncateContents)).toBe(
+      true
+    );
+    expect(
+      utils.blogPostHasTruncateMarker(blogPostWithoutTruncateContents)
+    ).toBe(false);
+  });
+
+  test('extractBlogPostBeforeTruncate', () => {
+    expect(
+      utils.extractBlogPostBeforeTruncate(blogPostWithTruncateContents)
+    ).toMatchSnapshot();
+    expect(
+      utils.extractBlogPostBeforeTruncate(blogPostWithoutTruncateContents)
+    ).toMatchSnapshot();
+  });
+
+  test('extractBlogPostSummary', () => {
+    expect(
+      utils.extractBlogPostSummary(blogPostWithTruncateContents)
+    ).toMatchSnapshot();
+    expect(
+      utils.extractBlogPostSummary(blogPostWithoutTruncateContents)
+    ).toMatchSnapshot();
+  });
+});