Lots of misc

[shellscape]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

• Fixed bug where a partial config, relying on webpack@4 defaults, would fail if entry was omiited
• Updated the README with docs for on and add
• Improved event bus test coverage. Now throws in invalid options instead of silently failing.
• Fixed issue with get-port where it would sometimes return a port in use

Breaking Changes

The event bus will now throw on invalid options.

Additional Info

This will be the last maintenance PR before v0.2.0

[codecov]

Codecov Report

Merging #47 into master will increase coverage by 0.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
+ Coverage   91.89%   92.04%   +0.15%     
==========================================
  Files           7        7              
  Lines         259      264       +5     
==========================================
+ Hits          238      243       +5     
  Misses         21       21

Impacted Files	Coverage Δ
lib/bus.js	100% <100%> (ø)	⬆️
lib/server.js	87.69% <100%> (ø)	⬆️
lib/options.js	87.01% <100%> (-0.33%)	⬇️
lib/config.js	100% <100%> (ø)	⬆️

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