Add a config:install command

[pjcdawkins]

• Implement install of config file and executable script
• Implement auto update of config file
• Add confirmation question

Tested manually on Ubuntu, Windows, Fedora and Arch

Auxillary changes:

• The CLI's version number is moved from commands.version to config.Version so that it can be reused in a cnf.UserAgent() method.
• This is based on (includes) the following MRs (with only minor conflicts, if merged separately):
  • Rework error and debug handling #228 to show errors
  • Use Masterminds/semver package to compare versions #229 so that version comparison and validation can be reused
  • Fix help for Cobra commands #230 to show help for the command's flags
• The confirmation question uses https://github.com/symfony-cli/terminal, so I'm also reusing the terminal.IsTerminal function in updates.go.