feat: add build-started and build-finished events

[shellscape]

This PR contains a:

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

Motivation / Use-Case

Users have requested events to be notified when a build starts and ends.

Breaking Changes

None

Additional Info

[codecov]

Codecov Report

Merging #122 into master will increase coverage by 0.4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #122     +/-   ##
=======================================
+ Coverage   92.59%    93%   +0.4%     
=======================================
  Files           7      7             
  Lines         297    300      +3     
=======================================
+ Hits          275    279      +4     
+ Misses         22     21      -1

Impacted Files	Coverage Δ
index.js	98.18% <100%> (+2.02%)	⬆️

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[elisherer]

Can I apply plugins to the compiler on 'build-start', or should It be done on "before-build-start"?
I'm asking because that way I could change webpack-serve-waitpage to apply the ProgressPlugin on it's own without asking the developer to do it.

Nevermind, just realized I could get the compiler(s) from the options passed to add.