Feat/add required flags

[HackAfro]

Fixes #51.

This pull request attempts adding the feature to enable required flags.

Examples:

1. Making a flag required:

const cli = meow({
	description: 'Custom description',
	help: `
		Usage
		  foo <input>
  `,
	flags: {
		foo: {
			alias: 'o',
			required: true
		}
	}
});

Output without required flag:

Missing required option
  --foo, -o

Run with --help to view help information.

2. Creating provisional required flags. These flags are only required if another flag is present.

const cli = meow({
	description: 'Custom description',
	help: `
		Usage
		  foo <input>
  `,
	flags: {
		unicorn: {alias: 'u'},
		fence: {
			alias: 'f',
			required(args) {
				return args.unicorn;
			}
		},
	}
});

fixture --unicorn="horns"

The command above would make fence a required flag. Meaning fence is only required if unicorn is present.

[sindresorhus]

Please don't do unrelated changes. Revert the Prettier changes. Unrelated changes makes it very hard to review the actual changes.

[sindresorhus]

Missing required option

I think that should be option => flag

[sindresorhus]

From #51 (comment):

I also think we should call the option isRequired, not required.

[sindresorhus]

This is also unanswered:

Note, I'm not exactly sure what the function arguments should be. I'm open to suggestions there. I guess the input and flags. Anything else?

[sindresorhus]

It would be better to create a new fixture file for just testing the required flag stuff.

[HackAfro]

Okay. Will include this

[sindresorhus]

The title needs to be clearer. I'm not sure what exactly is being tested here.

[HackAfro]

The test is for optional required flags. Will update the description to make it clearer

[sindresorhus]

You also need a test for the positive side, when --fence is actually given.

[sindresorhus]

It would be better to test only the important part of the message, so other parts are free to change in the future without having to update all the tests. You can use t.regex instead.

All we need to test is Missing required option:\n --foo, -o.

[sindresorhus]

Use t.regex here to simplify the assertion.

[HackAfro]

This is also unanswered:

Note, I'm not exactly sure what the function arguments should be. I'm open to suggestions there. I guess the input and flags. Anything else?

I think the function arguments should be the flags or the args rather. Allowing the user to check the availability of any of the inputted flags

[sindresorhus]

@HackAfro Still interested in finishing this?

[HackAfro]

Yes. Let me make these changes and update the PR

[sindresorhus]

@HackAfro Bump

[sindresorhus]

Closing for lack of response.