feat: Add subtitle option

[dusktreader]

It would be really nice to be able to include a subtitle as well. This would be rendered on the bottom border. It could be configured exactly the same as the title.

Example:

┌ species ───────┐
│                │
│ jawa ewok hutt │
│                │
└ (four letters) ┘

[sindresorhus]

I wouldn't call that subtitle, rather footer.

[dusktreader]

I wouldn't call that subtitle, rather footer.

I didn't see this till I made the PR.

Something to consider though: "footer" implies a "header", while "subtitle" implies a "title".

[sindresorhus]

subtitle is wrong here. Subtitles go below titles but still at the top, not at the bottom of a container. This text literally appears at the bottom border, making it a footer by definition. The fact that the top option is called title doesn't change what we should call bottom content.

[dusktreader]

subtitle is wrong here. Subtitles go below titles but still at the top, not at the bottom of a container. This text literally appears at the bottom border, making it a footer by definition. The fact that the top option is called title doesn't change what we should call bottom content.

Renamed to "footer" in latest commit