Allow different env vars for each command

[Coridyn]

This allows you to pass different environment variables to each command.

This PR adds an optional env property to the commands object.

The values on the env object take precedence over process.env so it can override any global environment variables that may have been set earlier.

[coveralls]

Pull Request Test Coverage Report for Build 332

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 99.183%

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Totals
Change from base Build 331:	0.004%
Covered Lines:	238
Relevant Lines:	238

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 332

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 99.183%

---

Totals
Change from base Build 331:	0.004%
Covered Lines:	238
Relevant Lines:	238

---

💛 - Coveralls

[coveralls]

Coverage remained the same at 100.0% when pulling 0e0f8b3 on Coridyn:master into a0cc081 on kimmobrunfeldt:master.

[gustavohenke]

Published v5.2.0 with this 🚢