Refactor code in /lib to conform to ESLint (JavaScript the Good Parts)

[nelsonic]

Due to the lack of a Lint configuration the code had become inconsistent.
So this PR:

• added the goodparts ESLint module
• linted the code in /lib to conform.
• added JSDoc-style comments (code documentation) to all the major methods
  (hopefully making them easier to understand)
• fixes As part of improving code-readability can we also conform to ESLint (e.g: goodparts) ? #211 😉

Not made any changes to the test(s) which confirms that this PR is a straight re-factor with no addition/loss of features (100% compatible) compared to previous version(s) of the plugin.

It wasn't this bad ...

But we needed something to tidy up the code.

The Good News is that with reduction in Cyclomatic Complexity (enforced by goodparts), the plugin runs marginally faster in our ("informal") benchmarks.

Thanks again to all the people who are using this plugin in their apps ✅
and especially to all who have contributed improvements! ❤️

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #213 into master will not change coverage

@@           master   #213   diff @@
====================================
  Files           2      2          
  Lines          94    108    +14   
  Methods         0      0          
  Messages        0      0          
  Branches        0      0          
====================================
+ Hits           94    108    +14   
  Misses          0      0          
  Partials        0      0          

Powered by Codecov. Last update 2c158ab...6eac431

[iteles]

This is some pretty impressive image usage for a PR description 😆 Do we want to also bump the version number?

[nelsonic]

@iteles after the PR is merged I run the following command(s):

git checkout master
npm version patch -m 'lint code using goodparts see: https://github.com/dwyl/hapi-auth-jwt2/issues/211 -> 100% backward compatible no features added/removed.'
npm publish && git push && git push --tags

see: https://docs.npmjs.com/getting-started/publishing-npm-packages#updating-the-package