Switch dependency `mitchell/cli` to `hashicorp/cli`

[austinvalle]

Closes #110

One interesting tidbit, the tools module still has an indirect reference to mitchellh/cli because of github.com/hashicorp/copywrite.... The copywrite CLI doesn't actually use mitchell/cli directly but has an indirect reference to it as well 😄

The chain of dependency for both tools/go.sum references:

• github.com/mitchellh/cli@v1.1.0
• github.com/hashicorp/serf@v0.9.6
• github.com/hashicorp/consul/api@v1.13.0
• github.com/knadh/koanf@v1.5.0
• github.com/hashicorp/copywrite@v0.18.0 🎯

And the other:

• github.com/mitchellh/cli@v1.0.0
• github.com/hashicorp/go-sockaddr@v1.0.2
• github.com/hashicorp/vault/sdk@v0.1.13
• github.com/hashicorp/vault/api@v1.0.4
• github.com/knadh/koanf@v1.5.0
• github.com/hashicorp/copywrite@v0.18.0 🎯

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