Add support for gas reporting

[abcoathup]

Add support for gas reporting e.g. https://github.com/cgewecke/eth-gas-reporter

Requested in the Community Forum: https://forum.openzeppelin.com/t/openzeppelin-test-environment-gas-reporting/2524 (also previously: https://forum.openzeppelin.com/t/recommended-test-setup-with-oz-sdk/1790/6)

[jamesmorgan]

Is there any updates on this?

We would love to be able to easily profile my contracts using test-env - it being even more important these days within the current GAS climate as well.

[frangio]

@jamesmorgan We won't be able to work on this in the near future but we welcome contributions if someone wants to work on it.