New option: title

[Caesarovich]

Hi, would be ncie to have the possibility to add a title to a box. Like:

┌Tasks─────────────┐                                                                                                                                                                                                                                                                                       │Task number 1...                             │
└───────────────  ┘

I've already edited the source code to make it possible, I can open a PR.

[sindresorhus]

The codebase is already quite complex so I'm a little bit resistant to add more complexity, but I'm willing to have it added if it comes in the form of a high-quality pull request with tests, types, and docs.