Add onServerReady, onTestsComplete, onIstanbulComplete Hooks

[cgewecke]

These allow you to run preparatory scripts which require a connection to the ganache server before executing the tests. Launching the Oraclize/Provable is an example use-case. Terminal hooks allow you to clean resources up or post-process generated artifacts.

These hooks are meant to mitigate some of the problems raised by having an in-process client which cannot be launched separately at the command line. See openzeppelin 1918

[codecov-io]

Codecov Report

Merging #408 into truffle-plugin will increase coverage by 0.03%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           truffle-plugin     #408      +/-   ##
==================================================
+ Coverage           96.75%   96.78%   +0.03%     
==================================================
  Files                  14       14              
  Lines                 647      654       +7     
==================================================
+ Hits                  626      633       +7     
  Misses                 21       21

Impacted Files	Coverage Δ
lib/api.js	95.79% <100%> (+0.14%)	⬆️
dist/truffle.plugin.js	100% <100%> (ø)	⬆️

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