Improve test coverage

[brettz9]

This PR adds a few more tests to improve the test coverage of the plugin to 100%

[brettz9]

I've rebased and also bumped the version to drop Node 6 (the latest nyc I added requires it, it also would allow using up-to-date versions of Mocha and import-from, and even Node 8 has been end-of-lifed and being dropped by many projects).

[brettz9]

Thanks for the merge! I've rebased this one now as well...

[brettz9]

Was breakage with Mocha 8 and ESLint 3, so also dropped <=3 from peerDeps and Travis.

[brettz9]

And with Mocha 8 requiring Node 10 and Node 8 end-of-lifed, I added another commit to drop Node 8 from engines. If you still want the support, travis.yml should test Node 8 I think but it would also need to downgrade Mocha as well as ESLint.

[Turbo87]

@brettz9 can you split these changes into multiple PRs so that they can be discussed individually? 🙏

[brettz9]

Sure, I'm happy to split them up, and I started #114 which presents one item by itself.

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FWIW though, if you are interested to know my process on why I connected all of these...

I added nyc so one can get reports that all lines of your code base are covered by the tests. I set thresholds at 100% so any regressions of coverage would break Travis. And to avoid breaking Travis with such coverage now checked, I added tests necessary to bring the project to 100% coverage.

Since the latest nyc requires Node 8+, I updated travis.yml to drop Node 8 and check the current supported Node versions (12 and 14). And if you are ok upgrading, I figured we might as well also check ESLint 7 and Mocha 8 which depend on higher Node versions, though with Mocha 8, we have to drop ESLint 3 (as it fails with it) and bump Node further to Node 10+.

[brettz9]

Rebased also with no breaking changes.