Add prototype access proxy

[stephen-soltesz]

For example:

 ./access-proxy  -forward http://localhost:8080@http://localhost:8081

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This change is 

[stephen-soltesz]

FYI: @robertodauria this was an early proof of concept from May. Tested locally, but not on a real machine. May be helpful.

[coveralls]

Pull Request Test Coverage Report for Build 161

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.5%) to 99.057%

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Files with Coverage Reduction	New Missed Lines	%
cmd/envelope/main.go	3	97.93%

Totals
Change from base Build 152:	-0.5%
Covered Lines:	630
Relevant Lines:	636

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💛 - Coveralls