Added CHANGELOG.md generator script

[mauricetmeyer]

Closes #99

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

See sendgrid/smtpapi-nodejs#91 for a usage and format.

[childish-sambino]

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