Add shx.Job interface with State, Description, and ExecJob

[stephen-soltesz]

This change adds a new package shx that provides shell-like operations for Go. Ultimately, shx is intended to replace M-Lab's use of the m-lab/pipe package because this package is a modified fork of a 7 year old, unmaintained package from github.com/go-pipe/pipe with several limitations.

The shx package overcomes these limitations with the following features:

• Simpler implementation.
• Uses context.Context for job cancellation or limits.
• Provides full shell-like Description of jobs.
• Simple to define custom Job types that can be used with shx types.

This change is 1 of two or three to add more primitive types described in the package docstring: Exec, System, Func, Pipe, Script, with various convenience Jobs for common shell operations.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 997

• 115 of 120 (95.83%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 92.775%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
shx/job.go	115	120	95.83%
<!--	Total:	115	120	95.83%	-->

Totals
Change from base Build 967:	0.2%
Covered Lines:	1695
Relevant Lines:	1827

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💛 - Coveralls

[stephen-soltesz]

An example command using shx: https://github.com/m-lab/locate/blob/soltesz-shx-experiment/management/create_encrypted_signer_key.go

An example description of that command: https://github.com/m-lab/locate/blob/soltesz-shx-experiment/management/description_script.sh