Validate solcoverjs options with json-schema

[cgewecke]

Includes checks for to-be-implemented sequence hooks (functions):

onServerReady
onTestsComplete
onIstanbulComplete

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (truffle-plugin@0b8c303). Click here to learn what that means.
The diff coverage is 100%.

@@                Coverage Diff                @@
##             truffle-plugin     #403   +/-   ##
=================================================
  Coverage                  ?   96.82%           
=================================================
  Files                     ?       12           
  Lines                     ?      629           
  Branches                  ?        0           
=================================================
  Hits                      ?      609           
  Misses                    ?       20           
  Partials                  ?        0

Impacted Files	Coverage Δ
lib/ui.js	100% <ø> (ø)
lib/app.js	96.22% <100%> (ø)
lib/validator.js	100% <100%> (ø)

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