Bash and zsh auto completions

[Crevil]

This PR adds auto-completion to run scripts.

This is an example of the outputs in zsh.

$ shuttle run --[tab]
--clean      --plan       --project    --skip-pull  --template   --verbose
$ shuttle run [tab] 
release     test        test-go     test-shell
$ shuttle run t[tab]
$ shuttle run test
$ shuttle run test[tab]
test        test-go     test-shell

It works for script arguments as well.

scripts:
  test:
    args:
    - name: tag
    - name: version

$ shuttle run test [tab]
tag      version

Changes

• Custom completion function to the root command that will be called when ever the generated cobra completions don't have an answer.
  This is the case when calling the run command as we don't know at compile time what scripts will be available.

• template flag added to the ls command to allow for simple programatic lookups of scripts.

• cobra upgraded to commit spf13/cobra@d2d81d9 as there is a bug in the custome completion implementation (Fix too many underscore for __custom_func spf13/cobra#794). The bug is fixed but not released.

Closes #23

[kaspernissen]

This is awesome! 👏 Great job, @Crevil !
Not sure, if I'm the best to review this PR, but from what I can see, it looks good!