Added CHANGELOG.md generator script

[mauricetmeyer]

Closes #90

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• This PR adds a generator script to update the CHANGELOG.md

Usage example:

$ GITHUB_TOKEN=... script/update-changelog 4.0.0

This will add a new entry in the following format:

[4.0.0] - 2018-10-10

Added

• PR #80: Update LICENSE.txt. Thanks uppe-r for the PR!
• PR #66: Update .codeclimate.yml. Thanks Matt Bernier for the PR!
• PR #52: add usage.md, issue #48. Thanks Rohit Daryanani for the PR!
• PR #64: Added TROUBLESHOOT.md, fixes #47. Thanks dave4506 for the PR!
• PR #62: Add a unittest to check the license.md file date range, fixes #57. Thanks Varayut Lerdkanlayanawat for the PR!
• PR #53: Added esdoc support, fixes #50. Thanks Manjiri Tapaswi for the PR!
• PR #55: Create .codeclimate.yml, this pr fixes #54. Thanks Prempal Singh for the PR!
• PR #59: Update License date range, fix #56. Thanks Camilo Cortes for the PR!
• PR #49: Add a .env_sample file, update gitignore, update README.md, issue #45. Thanks thepriefy for the PR!
• PR #34: Added Troubleshooting.md, in reference to #26, i've added a getter function to the header lib, which could be used by the developer, by console.log({headers: headers.get()}). Thanks Akram Rameez for the PR!
• PR #63: Update the license file to have the correct date range, fixed #56. Thanks Varayut Lerdkanlayanawat for the PR!
• PR #60: Added unittest to check for specific repo files, fixes # #58. Thanks Manjiri Tapaswi for the PR!
• PR #44: Add Pull Request template file, closes #41. one of my first few open source additions, let me know if i need to fix anything. happy hacktoberfest!. Thanks Jeff Gerlach for the PR!
• PR #38: Freeze dev dependencies so tests pass on 10.x. Thanks Martin Gingras for the PR!
• PR #40: add .github/ISSUE_TEMPLATE. Thanks thepriefy for the PR!
• PR #37: Updated travis config. Thanks Shivam Agarwal for the PR!
• PR #35: add table of contents in README.md. Thanks thepriefy for the PR!
• PR #33: Making README/Doc Sections More SEO Friendly, closes #28. Thanks Martin Gingras for the PR!
• PR #31: Update README.md - moved logo on top and added more badges, #29. Thanks Alex for the PR!
• PR #30: SEO Friendly Section links, updates for seo-ness (sendgrid/smtpapi-nodejs#28). Thanks Eric Liu for the PR!
• PR #22: Update README.md. Thanks Cícero Pablo for the PR!
• PR #24: Added Code of Conduct, solves the issue #23. Thanks Shivam Agarwal for the PR!

Fixed

• PR #39: update contributing and readme - fix typo and ToC. Thanks Alex for the PR!
• PR #27: Fix grammar. Thanks Aaron Ang for the PR!

[childish-sambino]

Closing as we're now using internal tooling to automate this.