Uses the github.com/ghodss/yaml library to convert YAML to JSON before
unmarshaling for the purposes of supporting YAML and JSON input while
converting to only support JSON internally.

Bug: T207694
Change-Id: I00668014907e9ea54917f5d5067cac08d0668053

Summary:
Wrote inline documentation for all the things.

Fixes T168000

Test Plan: Execute `godoc -http=:6060` and proofread all the things.

Reviewers: thcipriani, hashar, #release-engineering-team, demon

Reviewed By: thcipriani, #release-engineering-team, demon

Tags: #release-engineering-team

Maniphest Tasks: T168000

Differential Revision: https://phabricator.wikimedia.org/D841

Summary:
One of Golang's "advantages" is not quibbling over style. To this end
the `go fmt` command exists. This is the result of me running:

    find . -name '*.go' -exec go fmt {} \;

Test Plan: built package, ran tests

Reviewers: dduvall, mmodell, #release-engineering-team

Reviewed By: dduvall, mmodell, #release-engineering-team

Subscribers: hashar

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D694

Summary:
Refactored bool config fields to use a new `config.Flag` type that keeps
track of whether it was set by unmarshalled data or merged from another
instance, fixing the behavior of these fields when overwritten by
variants.

Fixes T166353

Test Plan: Run config unit tests (`cd config; go test`)

Reviewers: thcipriani, Joe, hashar, mobrovac, mmodell, #release-engineering-team

Reviewed By: mobrovac

Tags: #release-engineering-team

Maniphest Tasks: T166353

Differential Revision: https://phabricator.wikimedia.org/D680