Add 'onCompileComplete' option

[cgewecke]

An async hook which runs after compilation, before the tests. It's passed the re-targeted config. It's necessary for projects like JoinColony which have secondary compilation steps and move some files around right before the tests run. (See their npm script step: provision:token:contracts)

Also fixes a couple bugs in the UI output and adds an entry for --network in the CLI options section of the README

[codecov-io]

Codecov Report

Merging #413 into truffle-plugin will increase coverage by <.01%.
The diff coverage is 100%.

@@                Coverage Diff                 @@
##           truffle-plugin     #413      +/-   ##
==================================================
+ Coverage           99.05%   99.06%   +<.01%     
==================================================
  Files                  14       14              
  Lines                 636      639       +3     
==================================================
+ Hits                  630      633       +3     
  Misses                  6        6

Impacted Files	Coverage Δ
dist/plugin-assets/truffle.ui.js	100% <100%> (ø)	⬆️
lib/api.js	100% <100%> (ø)	⬆️
dist/truffle.plugin.js	100% <100%> (ø)	⬆️

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