Adds shell completions and custom user-agent

[displague]

• Adds shell completions source <(packet completion bash) / eval "$(packet completion bash)"
• Adds plural and singular command aliases for each command (this is inconsistent between resource types, adding the alternates resolves this)
• Moves doc generation to a command packet docs ./docs (instead of requiring an environment variable to be set)
• Includes packet-cli version in the User-Agent of the API requests

Closes #55
Closes #56
Closes #63

---

$ source <(packet completion bash)
$ # in Bash 3.2.x, you may need to use:  eval "$(packet completion bash)"
$ packet <tab>
2fa                   ip                    user
capacity              operating-systems     virtual-network
device                organization          volume
event                 plan                  vpn
facilities            project

$ packet device <tab>
create  delete  get     reboot  start   stop    update

$ packet device get --<tab>
--config       --config=      --id           --id=          --json
--project-id   --project-id=  --yaml

[jasmingacic]

@displague I will merge this