This adds subcommand "completion" to the CLI which:
- generates completion script: ginkgo completion bash
- installs completion script: ginkgo completion --install bash
- generates commands completion: ginkgo completion --complete bash --
- generates flags completion: ginkgo completion --complete bash -- run --
- without suggests script fails back to default and completes file paths

For "bash" use hack similar to used in cobra, but implement it in go:
- single suggests are issued as is without description
- multiple suggests are issued in form "{suggest}{align}({description})"
I.e. description is a part of suggest but shell will use only common prefix.

For "zsh" issue completion response in form "{suggest}:{description}".

For "fish" issue completion response in form "{suggest}\t{description}".

Signed-off-by: Konstantin Khlebnikov <[email protected]>