Added CHANGELOG.md generator script

[mauricetmeyer]

Closes #115

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.

[codecov]

Codecov Report

Merging #117 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #117   +/-   ##
======================================
  Coverage    84.7%   84.7%           
======================================
  Files           3       3           
  Lines         170     170           
======================================
  Hits          144     144           
  Misses         26      26

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[childish-sambino]

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