Set NODE_ENV if unset at runtime – or – if set, use it

[mstade]

Description

When the --production flag is set, we ensure ez-build also sets NODE_ENV internally, such that build setups that use process.env.NODE_ENV substitutions work as expected. However, if NODE_ENV is already set, we leave it alone. Additionally, if NODE_ENV is set to production prior to invoking ez-build, it too will enable production mode. This means it's possible to enable production mode in the following ways:

• ez-build --production
• NODE_ENV=production ez-build
• npm run --production build where build is a script that invokes ez-build

  Motivation and Context

More color on this topic is found in #38, which is fixed by this PR.

How Was This Tested?

Both unit tests and CLI tests were added, to test for options parsing and changes to build execution.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

  Checklist:

• [x] My change follows the style of this project
• [x] My change requires a change to the documentation
• [x] I have updated the documentation accordingly
• [x] I have read the contribution guidelines
• [x] I have added tests to cover my changes
• [x] All new and existing tests passed

[codecov-io]

Current coverage is 86.89% (diff: 100%)

Merging #42 into master will increase coverage by 0.19%

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  Methods           0          0          
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+ Hits            228        232     +4   
  Misses           35         35          
  Partials          0          0          

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