633 add support for user defined commands

[wd60622]

Describe what this PR does / why we need it

This allows users to pass additional commands to their config

Includes:

• autocomplete in :Octo <tab>
• Added to picker with :Octo<CR>

require("octo").setup {
  ..., 
  commands = {
    auth = {
      status = function()
        local output = vim.fn.system("gh auth status --active")
        vim.notify(output)
      end, 
    },
  },
}

Related to #743 CC @uwla

Does this pull request fix one issue?

Describe how you did it

Extend the M.commands table at setup with new configuration option commands which has a default of {}

Describe how to verify it

1. Add some new command to your configuration
2. Try the autocomplete with :Octo <tab>
3. Try the picker with :Octo<CR>

Special notes for reviews

Checklist

• Passing tests and linting standards
• Documentation updates in README.md and doc/octo.txt