If plugin is disabled, we should not modify the html at all

[AnujRNair]

Summary

When a CSP plugin instance is disabled, we should not modify the HTML at all, as proposed in this PR comment: https://github.com/slackhq/csp-html-webpack-plugin/pull/16#issuecomment-444726998

This means that cheerio won't add empty HTML tags to the file it is modifying, potentially causing incorrect HTML markup

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 #28 into master-v3 will decrease coverage by 0.1%. The diff coverage is 100%.

@@             Coverage Diff              @@
##           master-v3     #28      +/-   ##
============================================
- Coverage      93.91%   93.8%   -0.11%     
============================================
  Files              2       2              
  Lines            115     113       -2     
  Branches          21      21              
============================================
- Hits             108     106       -2     
  Misses             6       6              
  Partials           1       1

Impacted Files	Coverage Δ
plugin.js	92.3% <100%> (-0.17%)	:arrow_down:

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