Ensuring that html webpack plugin hooks are available

[AnujRNair]

Summary

Ensuring that html webpack plugin hooks are available before attempting to register callbacks on hooks - if html webpack plugin has not been defined, this will error out

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've written tests to cover the new code and functionality included in this PR.
• [x] I've read, agree to, and signed the Contributor License Agreement (CLA).

[codecov[bot]]

Codecov Report

Merging #39 into master will increase coverage by 0.05%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage    93.8%   93.85%   +0.05%     
==========================================
  Files           2        2              
  Lines         113      114       +1     
  Branches       21       22       +1     
==========================================
+ Hits          106      107       +1     
  Misses          6        6              
  Partials        1        1

Impacted Files	Coverage Δ
plugin.js	92.39% <100%> (+0.08%)	:arrow_up:

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