Begin refactoring of lb commands

[jpignata]

This pull refactors a subset of the lb commands and the underlying pkgs used to implement them including alias, create, and list. This is quite a large pull, and the work isn't complete, but it's beyond the size I'm comfortable with remaining pending in a topic branch, so I want to merge it in and continue the refactoring in smaller pulls.

No significant behavior changes are included in this pull. This pull only introduces changes required for test coverage, those tests, and output and code style changes.

• Add interfaces for route53, elbv2, and ec2 clients and begin adding test coverage and godoc
• Adopt golang style for initialisms
• Explicitly pass Output struct to embedded operation structs rather than in functions
• Fetch load balancer listeners concurrently in lb list
• Bundle flag variables into a single global struct to reduce the surface area of what's exposed globally and to use the compiled to check for typos
• Add -cover parameter to make test in Makefile
• Split Titleize and Humanize string utils
• Extract port-related code into port file and refactor
• Prefix each error with a hyphen in fatal output
• Tweak overall output style for consistency and brevity