A helper method to prompt the user for some detail
If the `--prompt` arg is specified, prompt the user for arguments
Register `--prompt` as a global argument

Doing so is more correct, and avoids this warning:

```
Warning: unknown --prompt parameter
```
Properly set positional arguments
Merge branch 'master' into prompt-673
Use `\cli\prompt()` instead of reinventing the wheel

See https://github.com/wp-cli/wp-cli/pull/673#issuecomment-22938858
Support for repeating positional arguments
Better use of `\cli\prompt()`, which requires a default argument if the prompt is to be optional
Add support for prompting `--flag`
Support for generic arguments
A better way of achieving this.

See https://github.com/wp-cli/wp-cli/pull/673/files#r5969302
Catch the thrown exception
We don't need a `prompt()` util right now. Instead, let's better utilize PHP cli tools
remove leftover $prompt property
Merge pull request #673 from wp-cli/prompt-673

Global support for `--prompt`