Add --temp command flag

[cgewecke]

Specifies the name of the folder to write compilation artifacts to. A use case is colonyNetwork where there are complex scripts to set up the tests which consume various pieces of the Truffle artifact. All this logic assumes they exist underbuild (rather than .coverage_artifacts).

This folder gets created/deleted and needs to be disposable. The user cannot store meaningful deployment data in it.

[codecov-io]

Codecov Report

Merging #414 into truffle-plugin will not change coverage.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           truffle-plugin     #414   +/-   ##
===============================================
  Coverage           99.06%   99.06%           
===============================================
  Files                  14       14           
  Lines                 639      639           
===============================================
  Hits                  633      633           
  Misses                  6        6

Impacted Files	Coverage Δ
dist/plugin-assets/plugin.utils.js	98.48% <100%> (ø)	⬆️

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