Allow enabling separate pieces of functionality using feature flags in the configuration

We should be able to enable/disable specific commands as "feature flags", these feature flags are configured in a yaml config file.

If the feature is not enabled, we fall back to ruby. If the config can't be read, we fall back to ruby.

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Original:

Split out from #74 (closed)

We have an ambition to start rewriting gitlab-shell in Go. We intend to do this in an incremental fashion, instead of as a big bang.

One important component that all the gitlab-shell tasks depend on is an API client to the internal GitLab API endpoints. This is being tackled as a component of its own by @reprazent and @zj at present, so needs to be its own issue.