Add title and titleAlignment options

[Caesarovich]

This PR adds a new feature allowing users to add titles to their boxes.
That is a very handful feature for building CLI Tools.

See example below:

const title = 'Beautiful title';
console.log('\n\n' + boxen('This box has a nice title', {title}) + '\n');
console.log('\n\n' + boxen('This box has a centered title', {title, titleAlign: 'center'}) + '\n');

// Output:

┌ Beautiful title ────────┐
│This box has a nice title│
└─────────────────────────┘

┌────── Beautiful title ──────┐
│This box has a centered title│
└─────────────────────────────┘

• The titles can be left/center/right aligned.
• The titles supports custom coloration (such as chalk...).
• Titles are sliced if it were to exceed the terminal's size.
• In this limit, the boxes width are extended to accommodate the title's size as much as possible.

Fixes #56

[sindresorhus]

The options need to be documented in the readme too.

[Caesarovich]

I have effectuated the requested changes. I would like your point of view on the README especially.

[Caesarovich]

It should be good now, sorry again for the typo and english errors.

[sindresorhus]

Looks good. Thank you 🙏

Do you plan anymore changes or should I do a new release?

[Caesarovich]

Looks good. Thank you 🙏

Do you plan anymore changes or should I do a new release?

Hi, I am working on another branch to add the 'width' option. But it is not ready yet so you can go on 👌